The file structure for the Canadian Railways Set is based on the format for the Central European Train Set.

1. File name format/file location is:
src/gfx/A/B_C_D_E_F.png

where:
A is the main company name (e.g. "CNR").
B is the vehicle's "short name". For Steam Locomotives this will be Wheel Arrangement in Whyte Notation (e.g. "484"), Diesels and Electrics it will be model/Type (e.g. SD40-2) . 
C is the length in 4 px units (e.g. "32 px would be 8"). 
For Steam Locomotives, tenders may appear on the same spritesheet.  The Length of the Locomotive will be denoded with an L before the number, and Tender length will have a T (e.g. "L8_T6")
D is for variants on livery (e.g. 1st is "1", next is "2"... etc).  This may also be followed by Class in the case of Steam Locomotives. (alphanumeric and underscore only [A-Za-z0-9_], e.g. "1_U2a, 2_U4a").
 E = color depth (8bpp/32bpp). To begin with, all files are 8bpp.
F = zoom level (zi4, zi2, normal, zo2, zo4, zo8; only "normal" used at the moment)
The examples given above will produce a relative path of "src/graphics/CNR/484_8L_6T_2_U4a_8bpp_normal.png".

Be aware of case sensitivity - check the extension is ".png" and not ".PNG". The variation name (D) will be noted in the "Graphics" column in the tracking table.