Self propelled front loader

[Sumpter250]

I would just need to fasten some sort of guides for the front wheels so they don't veer away from the rails.

Seriously, just look at any auto, or truck, that a railroad would use for "track inspection", or "drive"on the track. They have wheel sets ( front and back ) that are lowered to the rails, and locked in place. They can then be raised and locked for highway driving.
Not only would they be "guides", they would be electrical pick-ups.
https://www.google.com/search?q=Track+in...d=0CD4QsAQ
Also:http://www.google.com/patents/US6988451 might give you some ideas.

I usually do not "inhabit the internet", but.......there is a world of research out there, and sometimes answers can be found.

Pete