Hi, Beef.
Dont know whot to write about this problem. I have installed:
CoFHCore [1.7.10]3.0.2-262
ThermalFoundation [1.7.10]1.0.0-81
ThermalExpansion [1.7.10]4.0.1-182
and
Railcraft [9.6.1.0]

They work fine on server and client-side. When I trying add BigReactors 0.4.3A (or any older version), I have a crash: http://pastebin.com/et6VJZ4y
looks like TE and RC can not share block of steam. I cant find any ID's of Steam fluid in configs TE\RC\BR. How to fix that?

What's your forge version, there are some fluid registry changes with newer versions of forge: try 1291. If not-well, update the rest of your mods??
Nvr mind, "There is a quick fix by editing the file ~/config/railcraft/blocks.cfg ( ~ is the directory your client/server runs from )

Change the line that reads "B:fluid.steam=true" to false and give it a test run.

When either mod pushes a new update that resolves this problem, it may be necessary to restore the originals values.

There is a thread on FTB that goes in to detail when dealing with conflicts: [Guide] Resolving Block Conflict IDs"
Quoted from http://www.minecraftforum.net/forums/support/modded-client-support/2413231-railcraft-crash