Back shop is usually referred to has a heavy locomotive rebuild shop; think Altoona Juniata, Silvis, etc. Historically boiler shops and fabrication. Running repairs, etc. were in a roundhouse, when it's time to tear down the boiler or modify a locomotive it goes to a backshop.
Car shops were usually a separate function physically, doing carbody repairs, truck swaps, etc. Biggest one ever is down in Hollidaysburg, PA (ex-PRR, PC, Conrail, now Curry Rail Services). Juniata didn't mess with cars, Hollidaysburg didn't mess with locomotives.
Diesels required such vastly different services that most, but not all railroads, simply started over with a new building entirely for diesel work. There are a few that didn't - Hornell on the Erie (now Alstom), Huron, SD on the C&NW (roundhouse now a diesel shop with the turntable), but most railroads elected to call a bulldozer and build new. But still, you'll see a rather sharp distinction between a car shop and a locomotive (back) shop. The skill sets, parts, etc. are pretty much separate skills.
For modeling that Walthers car / locomotive shop building is pretty good for kitbash material. It would be even better if you had room to put two of them end to end or side by side. Note that the 'high bay' on those is primarily for the internal craneway, typical in major locomotive shop but not necessarily typical in a car shop - you don't need to lift cars up sideways between bays like a locomotive. Our local car shop has only a relatively small craneway in the roof to move material but air car jacks everywhere.
About the 70's is when car and locomotive painting got serious. Before you just spray-gunned whatever and away you went. Now it's filtered, sealed, specialized. I've been in the paint shop in Huntington (CSX) and it's quite the operation, hospital clean and employee protection at the top. Back in the day our local car shop had a paint building and it was condemned by OSHA and the environmental guys. They don't paint cars now except for patching. Industrial painting has become rather problematic in general with VOC's.
There are a lot of independent car repair shops out there and a few locomotive rebuilders as well, those make really interesting industries in the modern era. We've got a local car repair shop and they specialize in repairing really damaged salt covered hoppers. But they'll do anything but tank cars.
