As highlighted above, 3 is the maximum number of special weapons they can take. Also, as darkstrider is technically embarked upon a transport, none of his aura effects work.
Railfinders are some of my favourite models, but the current meta has no room for them. The extra points make an already priority target almost certain to be wiped off the board first turn.
Hopefully post codex, there will be some new ways for pathfinders to be more durable and make the special weapon squads a viable option (maybe a +2 to saves when in cover?)
Until then, the 81 points for 3 railfinders is almost half the points for a fusion commander, which while lacking mortal wounds will be our ever reliable index anti-armour unit...