I usually use the map specific trains, and I don't use duals unless the map is specifically designed for them (B&R) or accomodates them well nonetheless (Arizona Gold is the only map I've added trains to, specifically a 4-8-2 Mountain and the F3 a&b unit). On other maps, the trains with duals are just too long; if you use duals on the firaxis maps, the trains stretch from one city to the next!
I prefer steam engines, which look better with a tender than they do doubled. However, some of the tenders from the TenderMod don't change colors with the train, so I've changed my trains XML file to only use the paintable tenders; I put a different tender behind the trains that had the other ones (ie the solid red Tendermod tender).
I wish we could get some more diversity in the diesels, we already have a F-series a&b unit, GP9, a GP38-2, and an AC4400, but I'd love to see a
F40PH, even if it was just the cab of the AC4400, shrunk down and grafted onto the F unit's body. An earlier GM would be nice too, either a Uboat or a dash 7. I reskinned a GP38-2 into a conrail
B23-7, but it just didn't look right without the bulge in the back (Like what the AC4400 has.) It's possible to delete inappropriate elements by making them transparent (such as the bell, the horns, the fans) and you can make other elements less pronounced (modifying the texture files made the nose look stubbier, more like a GE's) but you can't add anything, and in the end you really need to know how to work with 3d modeling to get it right.
I also reskinned a pair of electrics to make the AEM7 (from the Re 6/6) and the GG1 (from the p5a). Both were released with my America map. They both came out OK, but not being able to change the models really limited what I was able to do.
I'm still eager to see the SW1 and Dr. Frag's NR Class, which hopefully will come out soon.