Every thing you need to know about plans
First off any plan you find can have errors on it. I was given a job to scan in a box full of odd size plans to put on an internal web, it was all changes made to a new diesel engine that was all made on a computer. That I remember was move a hole to a better place and a parts maker made 50 covers with the two logos in the wrong order. Things like this happen all the time and correction plans are made and put with the other plans.
The maker plans are also very big, 40" wide by what ever and a scale of 3/8" or 1/2" to the foot.
Most of what you can find are reduced by photo or scanner. New plans that are made by computer can output to any size, a big help, but if it was made before 1970 it was not made on a computer, after 2000 all most all would be computer made.
The following is for U.S. locomotive and rolling stock.
You can find but costly and very big to work with.
Steam locomotives - normally shown with wheels and valve gear as center lines and diameter, no detail on side view. A lot of the piping is not shown as the pipe-fitters new what went where. Lots of sub-plans.
Diesel/electric locomotives - may or may not have bogeys.
Bogeys - side views for most can be found, top and front views very hard to find.
Freight cars - makers plans only show what they made not all the parts that they put on. So you find the body but no doors, brake gears, and all the other parts.
Passenger cars - the same as freight cars but always with a top view of the interior.
Railway Service cars (non-revenue and MoY} - same as freight cars but harder to find.
All cars show the bottom view as seen from the TOP, look out for this as it can get confusing.
You will find a lot of cutaways and cross sections.
Copy's of originals :
Found in Cyclopedias and trade magazines.
All printed copies can have have up to a 2% error, that's around one foot for a 85' car. You can also get the size wrong in ether X or Y direction.
Railroad Cyclopedias cab be found on-line, CD's and used books. Note, most do not show lettering or color.
Model Railway Magazines: First model railroad magazine came out in the 1930's and there have been a lot of them so I will only cover Model Railroader and Railroad Model Craftsman.
Both used plans made by users and staff and a lot of them from cyclopedias but they look better as the cut-a-ways are not there. Most printed after 1960 are good but the older ones can be missing things you need like the wheels.
All the newer ones normally all lettering, running numbers, and colors.
Size: Most are to model scales, S, HO, and O. RMC did try to use N but only for one year, to small to use. S scale fit better so most common.
Here is an old plane and you can see why you can not make a detailed model with just them.
Classification books: Your bigger railroad had them and I do not know just how they used them but you can find scanned ones on the internet. Most look hand drawn and do have good info. I have used them to make content.
Other: You can find plans in train books and on the internet. Look out for things that never got made, I know of a 4-8-6 plan to replace the 4-8-4 (Lima I think).