It'll be a code mod, so incompatible with any server-side mod such as JA+, MB2, etc.
Only available for MP, as we don't have the ability to create code mods for SP.
I'm just about done - I've replaced the rocket launcher, modified the missile impact function to stick to the player and detonate x milliseconds after impact, I just have to improve the client-side prediction, and perhaps play a sound whilst the rocket is inside them.
I think there was also a minor bug regarding NPC's. Not sure when it'll be ready for a release.
I'm no artist, so I can't model a rail detonator.