i'm trying to bulldoze all the rail behind my train but it just won't move forward because i can't place rail on already existing rail.

I'm assuming in Bulldozer mode? Try removing 1-2 rails in front of the train before starting it, that should work.
If it doesn't i need a save file.

it works on vertical rail, but when i do that on diagonal rail it just lays rail and deactivates; did you try Starting the Bulldozer on diag rail?

Ok, there seems to be a slight issue. Somehow it's not cleaning up enough in front.
Interestingly enough it works if you read a very simple diagonal blueprint into FARL. Just a rail and electric pole, e.g.

0eNqFkN0KAiEQhd9lrl1ICyxfJSL2Z9gGXBV1o2Xx3VODWArqSsY55ztHV+j0jM6TiaBWoN6aAOq8QqDRtLrcxcUhKKCIEzAw7VQm35KGxIDMgA9QPLG/lo7GBjX20VPfOKtx4xfpwgBNpEj4yq/DcjXz1KHPAW9MiDl7vMWmVmDgbMgua0puJjVZuZQjwwfyOa7u9qXgB1P8qPbFFRUrUila36U2P8fgjj5UsTjygzwJKXaSc5n1T2D3dKw=

made it work starting on diagonal rails. Edit: https://pastebin.com/BcNN2ULk
I'll see if i can fix it in the next few days, in the meantime try that blueprint or something similar. (You can even increase the area FARL clears by placing wooden chests in the blueprint, those will not be placed ever but increase the clearing area)

thank you chuo, this issue also extends to Maintenance (can't update poles on diag rails). also is there an option to autopilot cruise on already placed curved rails?; manual navigation is a bit tricky, and it seems cruise never makes turns (if not maybe you can make a 'lock-on-rail' option, but maybe that's too much work)

It should be fixed in the latest version.

By autopilot cruise you mean when in Bulldozer/Maintenance mode? I'm afraid not. I would love to add something like that (and a bunch of other features) but before adding anything new to it I need to rewrite it from scratch. The current code is so horrible and I don't understand most of it anymore :D