When using Liquid Fuelled Firebox with Boilers it is not possible to pipe in water with RP2 Fluid Pipes.
Piping in fluid fuels also doesn't work.

When using Solid Fuelled Firebox there is no problem.

Liquiducts from Thermal Expansion work.

RailCraft 1.46 - 6.12.0.0
Redopwer 2.0 PR6

So I didn't know if this issue is a problem with Railcraft or with Redpower but reporting a Redpower bug is not as easy.

EDIT:
Also when I try to pump with a RP2 Pump from an Iron Tank into the Boiler I get this Crash.
http://pastebin.com/20XQDuWq

Might be fixed in the next release.