REVISION
Sprites need to be ordered in a specific order before they can be coded. Of course you can put them in any messy order and let your coder figure out how to order them, but that's rather rude. As artist you know best what the order of sprites is, especially with complicated train consists you see in some multiple units. Below is an example of how sprites should be ordered.

The arrows show the driving direction of the vehicle. If your vehicle is symmetrical, such as a passenger coach, you can use 4 sprites. Otherwise you need 8 sprites like in the second row. Note that little details such as headlights, doors and panthographs can make your model unsymmetrical.
Please leave a gap of at least 2 pixels horizantally and 6 pixels vertically between your sprites, and be sure the upper row of pixels of all sprites on a row is nicely alligned, as well as the most left pixels of every row.
If necessary, comment your file about what every row of sprite is, for example the liveries, or head/middle/rear unit of a Multiple Unit.

Your coder really would appreciate that.