Feral Interactive’s support FAQ provides instructions for finding the UserMaps folder:

Custom maps, created using the terrain editor, are stored in your Preferences folder. To find them, follow these steps:

1. After creating and saving your map in Sid Meier’s Railroads!, exit the game.
2. Relaunch the game by double-clicking on the Sid Meier’s Railroads! Application icon while holding the Command (⌘) key. The Sid Meier’s Railroads! pre-game Options window will appear.
3. Select the Advanced tab and click on the Open Preferences Folder button. A new finder window will open showing the Sid Meier’s Railroads! folder contents on your computerâ€s hard drive.
4. Open the UserMaps folder to view a list of folders, each containing data for one of your saved custom maps.

— from http://support.feralinteractive.com/en/mac-games/railroads/faqs/share_maps/

Once you have the UserMaps folder open you must delete any other downloaded custom maps before installing a new custom map. Maps you have created yourself with the map editor do not need to be deleted, only delete maps you have downloaded from smrsimple.

Next unzip the downloaded map into the UserMaps folder. Now launch SMR and start a new game. At the map selection screen you may have to scroll through all the maps to find your new map.

Boring technical background:

The 3GB enabler is of script popular for games from the ~2006 era. This point in time marked a transition point as games ram requirements expanded to around 2GiB, a key limitation in 32bit Windows.  In 32bit systems the maximum amount of processor addressable space is 4GiB. In early 32bit versions of windows this space is split equally with Windows getting %50 and everything else getting the remaining %50.

As applications started becoming larger Microsoft made the limit more flexible giving Windows a smaller portion of the pie. Unfortunately this is not as simple as it should have been since many Windows Applications relied on this 2GiB cap. In order to maintain support for these programs Microsoft added a special value that programs could use to tell Windows that it is okay to give said program more than 2GiB.

What the 3GB Enabler does is add this special value to SMR’s railroads.exe which tells Windows to allow railroads.exe to use upto 4GiB of RAM.

Real Introduction:

Now this explanation is rather complex and might not interest you, the handsome, pragmatic user, who simply wants to play those large, awesome, custom maps available for download in the Map Database.

Fear not for I bring you a Simple! step by step guide complete with conditionals and my vain attempts at humor (Yes that is correct, I am not above self-deprecation).

Step 0:

Does your computer only have 2GB or RAM? Does your video card have ~2GB or RAM? Is SMR crashing with the stock maps?

If you said yes to any of these questions the 3Gb enabler cannot help you. I’m sorry but in order to enable more RAM you at least  need to have this more RAM stuff. In the case of high end video cards much of the extra space in 32bit address limit is being used up by the video card’s RAM. Lastly if SMR is failing to play the original maps this is indicates that some other bug is present.

You will not suffer any negative effects from the 3Gb Enabler but it does not bring any benifit.

Step 1:

Check if you have a 32bit or 64bit version of Windows, instructions courtesy of Microsoft:

To find out if your computer is running 32-bit or 64-bit Windows, do the following:

1. Open System by clicking the Start button , clicking Control Panel, clicking System and Maintenance, and then clicking System.

2. Under System, you can view the system type.

Step 2:

If you have a 32bit version of Windows XP you must do this:

Manually edit your boot.ini file by adding ” /3GB /Userva=2900″ to your boot.ini (no quotes).
Your boot.ini is normally read only and you will need to change that temporarily while you edit it.
To do this, right-click the file and untick “Read Only”. After editing the file, restore the Read-only status.
To Edit it manually you can find it on the root of your system drive or
go to system properties/advanced tab, settings under ‘startup and recovery†section, then hit edit.

Mine looks like:  multi(0)disk(0)rdisk(0)partition(1)\WINDOWS=”Windows XP Professional” /FASTDETECT /NOEXECUTE=OPTIN /3GB /Userva=2900

Step 3:

If you have a 32bit version of Windows Vista or Windows 7 you must:

Go to the start menu and in the search box type ‘cmdâ€, donâ€t hit enter. Wait until ‘cmd.exe.†appears on the menu and then right click and select “run as administrator”. Then run this command:  BCDEDIT.EXE /Set IncreaseUserVa 2900

Step 4:

Download this zip containing the 3GB Enabler script:  This link, right here.

Or download the pre-patched exe and replace the normal Railroads.exe, you may skip to step 5.5 then be done.

Step 5:

Extract the 3GB Enabler into the Sid Meier’s Railroad’s Program Files drictory. On 32bit versions of Windows this will be found at C:\Program Files\2K Games\Firaxis Games\Sid Meier’s Railroads!  In 64bit Windows: C:\Program Files (x86)\2K Games\Firaxis Games\Sid Meier’s Railroads!

Step 5.5:

If you have not already patched SMR to version 1.10 now is the time to do so. Without the 1.10 patch most custom maps will not work and many original maps will randomly crash. The SMR 1.10 patch can be found at Fraxis’ site: www.firaxis.com/games/game_detail.php?gameid=12&showcon=2

Step 6:

Inside the SMR Program files folder will be the files you extracted in step 5. One of these files is “startme.bat”, double click on this program to run it.

Step 6.1:

Hit the A key to add the large address header. Press any key to continue. You are now down and may close the window, or optional complete the next two steps to verify that the patch worked.

SMR Map Shifter C is a total redesign and rewrite in C++ of the original SMR Map Shifter program.

I focused on usability and zero-configuration, i.e. the program should work perfectly for all users and be intuitive to use. To that end it features only one button and no need to configure defaults. It also handles some rudimentary house cleaning steps to eliminate incorrectly installed maps.

For non-confusion’s sake this new version of SMR Map Shifter is version C named after the language it is written in C/C++, the old version is now retroactively SMR Map Shifter VB likewise named after programming language Visual Basic.

Instructions:

Press the single button, which will bring up a file selection dialog, select the map’s archive, now press Ok. The map will then be extracted and installed, you may now close SMR Map Shifter and run Sid Meier’s Railroads.

SMR Map Shifter’s Main window

File Selection Dialog

Warnning: This program works by deleting the files that currently reside in Usermaps and CustomAssets, do not use if you are creating a map or have other customized files saved in Usermaps. For the majority of users this is not the case.

Download:

SMR Map Shifter C version 1  installer in zip archive

General Fixes

Install Patch 1.10
Prior to 1.10 SMR was still a very unstable game. Updating SMR to 1.10 is critical. You do not need to install the 1.01 patch before 1.10 since 1.10 contains all the changes 1.01 had. Blame Firaxis for the close version numbers. If you were having problems and SMR was not updated to 1.10, then stop reading this article and update it. The 1.10 patch download.

3GB Enabler
Large maps logically use more RAM, unfortunately by default Railroads.exe only uses 2GB which means it will crash when it tries to use more than this. The 3GB Enabler allows Railroads.exe to use more than 2GB. If your experiencing crashes while playing large maps follow SMR Simple’s guide on installing the 3GB Enabler.

Empty the maps folder
Most maps cannot be installed while another custom map is present. This includes the ‘Holiday scenario’ official custom map. Make sure to clean out other custom maps before installing a new map, use the ‘How to empty “my games/sid meiers railroads”‘ guide.

Stock maps will crash while SAM or CIC maps are installed
If you have any SAM or CIC maps installed you will not be able to play the normal stock maps that originaly were included in SMR. To play stock maps again simply remove the SAM or CIC map.

Custom Map Fixes

Random Crashes due to Events.xml
Any map which uses a customized list of Goods must use an Events.xml which  does not contian any events that reference Goods which the map does not contian.  Should a random event come up which needs a good the map does not include in goods.xml the game will crash.  Because these crashes are random they are the most painful and mysterious to debug.

I came across the solution after Sam brought the quandary to my attention. The solution is actually quite simple, go to the start menu and in the search box type ‘cmd’, don’t hit enter. Wait until ‘cmd.exe.’ appears on the menu and then right click and select “run as administrator”. Then run this command:

BCDEDIT.EXE /Set IncreaseUserVa 2900

You can copy and paste that into the command prompt by using the right click menu, keyboard shortcuts CTRL+C and CTRL+V don’t work in the command line. Then restart for the changes to take effect.

Once you have done this and used the 3GB Enabler you should be able to play Sid Meiers Railroads crash free, so have fun and watch what time it is.

Note: Users of 64bit OSes don’t need to do this step and can use the 3GB enabler directly.

Luckily there is a fix for this. It is called the 3GB enabler and was originally made for another game that suffered from this bug, Stalker. I have edited the files so that they work with RailRoads.exe. Download the files from the bottom of this post and make sure you read the Readme I have copy pasted here:

Edit: If you are running the 32bit versions of Windows Vista or 7 you must skip the “Directions” section and instead follow this post to make 3GB enabler work under Vista & 7 due to the lack of boot.ini.

****** VERY IMPORTANT: IF YOU ARE USING AN x64 OPERATING SYSTEM: SKIP THE “DIRECTIONS” SECTION.

Introduction:

This is for intermediate to advanced Windows users.
Follow the instructions carefully…the risk is minimal, as long as you backup your RailRoads.exe.

This will enable RailRoads to use over 2GB of memory in the application space.  It uses conventional tools using a batch file so everyone can see nothing suspicious goes on.
This tool will automatically set the header in the RailRoads.exe so that it can address more than 2GB memory.
Normally XP will only address up to 2GB memory for applications and 2Gb for the kernel.
If we add a boot.ini switch /3GB, then XP is allowed to address up to 3GB for applications.
But unfortunately only applications that are enabled with “IMAGE_FILE_LARGE_ADDRESS_AWARE” header can utilize more than 2GB of the space.

I highly recommend you look at some official information here at microsoft:
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx

_______________________________________

Directions:

First you need to tell XP/Vista that it can use more than 2GB of RAM for applications.
Manually edit your boot.ini file by adding ” /3GB /Userva=2900″ to your boot.ini (no quotes).
Your boot.ini is normally read only and you will need to change that temporarily while you edit it.
To do this, right-click the file and untick “Read Only”. After editing the file, restore the Read-only status.
To Edit it manually you can find it on the root of your system drive or
go to system properties/advanced tab, settings under ‘startup and recovery’ section, then hit edit.

Mine looks like:  multi(0)disk(0)rdisk(0)partition(1)\WINDOWS=”Windows XP Professional” /FASTDETECT /NOEXECUTE=OPTIN /3GB /Userva=2900

Yours maybe different, the only thing you should change is add ” /3GB /Userva=2900″.

_______________________________________

Now for the tool to enable RailRoads to use more than 2GB…

*** Important: First make a backup original of you RailRoads.exe, by renaming the copy to Oblivion.exe.original ***

The tool will add the “LARGE_ADDRESS_AWARE” header for you to your executable.
Copy RailRoads.exe to this folder.
Doubleclick startme.bat then the menu walks you through.
1) Hit the A key to add the large address header.
2) Press any key to continue.
3) When back at main menu hit ‘V’ to output the headers in text so you can see the values.
4) Now you can check to see if it is enabled.  Scroll to the top and look for section “FILE HEADER VALUES”. Look for a line in that section that says “Application can handle large (>2GB) addresses”.  It’s the tenth line down in that section.  If you see that line then the executable is now enabled for >2GB, you are done.
5) Copy the enabled RailRoads.exe back to your Sid Meier’s Railroads! folder.
6) To avoid any strange issues with future patches, I suggest you rename your 3GB capable RailRoads.exe to something else like RailRoads3GB.exe, then make a shortcut to it on your desktop.  In the meanwhile, keep the original where it belongs and you will avoid any possible patching or online issues.

At anytime if you want to remove the large address header then you can restore you original backed up exe or use the app and use the Remove function and it will remove the header.  Once you run (R) to remove the header you have the original exe back, the MD5 hashes do match.

No strange side effects have been witnessed, but glad I can play some big games now.

Alright, since you have read the above this link will download the 3GB enabler edited for Sid Meiers Railroads. Do not forget to listen to the readme’s warning and skip the “directions” section of the instructions if you have a 64Bit operating system.

Once you have created your map do not forget to post it in the forum, so that it can be included in the map list.

Happy map making!