A simple mod which puts the diesel locomotive and cargo wagon attributes into a data file so that you can edit them as you see fit without editing your base game files. You can simply toggle the mod off to bring your train attributes back to the base values.
All attributes given in the mod are the same as vanilla, thus installing the mod initially has no effect.
All of the above mods have been run with this mod and don't seem to have any major conflicts, however the Hard Storage mod will override your changes to cargo wagon storage capacity in this mod. Solution: change the value in Hard Storage instead.
For later versions of Factorio, here is a different mod that accomplishes the same thing: