3 plans are available for the railway rifle.

free plan

MDPL-13 Power Station, go to Minefield and head NORTH-NORTH WESTtill you reach it, go inside and the plans are on the work bench.

Plan 2

Purchase from Tullip in Underworld at the General Store

Plan 3

Rewarded for completing Stealing Independence from the Museum in Rivet City

As for building it you'll need the following parts, 1 CRUTCH, 1 STEAM GAUGE ASSEMBLY, 1 FISSION BATTERYand1 PRESSURE COOKER

PS DONOT!!!!!!!!! kill the crazy sniper in minefield if you want to join the slavers in Paradice Falls as he is one of the targets they will send you to get, but it doesnt matter if you wanna kill the slavers, as his sniper rifle will come in handy (wink, wink)

And 1 last thing, check out ALL the power station buildings as it appers all are the same so its a good way to get cans of TURPENTINE, ABROXO CLEANER,1 BOTTLE OF NUKA-COLA QUANTUM and some TIN CANS (not the bent ones)for the NUKA grenade (very, VERY affective). AND you also get a free BOTTLE CAP MINEat the power stations, it will be on the work bench.