This happens in the middle of minecraft when playing. it seems like something is wrong with rail craft and IndustrialCraft. 
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Mods loaded: 50
ModLoader 1.0.0
mod_ModLoaderMp 1.0.0
mod_ReiMinimap v2.8_04 [1.0.0]
mod_TooManyItems 1.0.0 2011-11-29
mod_SpawnerGUI 1.0.0
mod_Stackables 1.0.0
mod_Somnia v17 [1.0.0]
mod_ModularForceFieldSystem 1.0 Beta 4_M1.0
mod_BCIC2Crossover 1.01 for MC 1.0, BC 2.2.8, IC2 1.42
mod_BuildCraftCore 2.2.8
mod_BuildCraftBuilders 2.2.8
mod_BuildCraftEnergy 2.2.8
mod_BuildCraftFactory 2.2.8
mod_BuildCraftTransport 2.2.8
mod_LogisticsPipes 0.1.2 (for Minecraft 1.0.0, Buildcraft 2.2.8, Forge 1.2.2)
mod_CraftGuide 1.2.2
mod_crystalWing Minecraft 1.0.0
mod_EE EEv5.7.3
mod_Forestry 1.2.4.3
mod_ForestryPlasticCraft 0.0.1
mod_IC2 v1.43
mod_MineFactory 1.0.0R1.4.0
mod_MineFactoryCompat_Grass 1.0.0R1.4.0
mod_IC2AdvancedMachines v3.0
mod_ChargingBench 1.42-1
mod_ElectricFishingPole 1.42-1
mod_IC2Thermometer v1.18
mod_ParticleDeco 1.1
mod_PortalGun 1.0.0v1
mod_PowerConverters 1.0.0R1.3.0
mod_Railcraft 2.1.1
mod_RedPowerArray 2.0pr4b
mod_RedPowerCore 2.0pr4b
mod_RedPowerLighting 2.0pr4b
mod_RedPowerLogic 2.0pr4b
mod_RedPowerMachine 2.0pr4b
mod_RedPowerWiring 2.0pr4b
mod_RedPowerWorld 2.0pr4b
mod_IC2_SolarHelmet v2.5
mod_ThaumCraft 1.0.2
mod_treecapitator 1.0.0 - v2 - DaftPVF
mod_WeaponMod 1.0.0 v8.1.0 Standard Edition
mod_WirelessClocker 0.3
mod_PowerConfigurator 0.6
mod_WirelessRedstone 1.5
mod_WirelessRemote 0.5
mod_WirelessSniffer 0.6
mod_WirelessTriangulator 0.5
mod_zAdditionalPipes Rev31.1-D
mod_CustomRecipes MC 1.0.0, mod subversion 0.1
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
If you wish to report this, please copy this entire text and email it to support@mojang.com.
Please include a description of what you did when the error occured.
--- BEGIN ERROR REPORT 57fd967c --------
Generated 23.12.2011 17:55
Minecraft: Minecraft 1.0.0
OS: Windows 7 (x86) version 6.1
Java: 1.7.0_01, Oracle Corporation
VM: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: GeForce GTX 280/PCI/SSE2/3DNOW! version 3.3.0, NVIDIA Corporation
java.lang.ClassCastException: railcraft.common.rails.TileRailSpeedSwitch cannot be cast to ic2.advancedmachines.TileEntityAdvancedMachine
at ic2.advancedmachines.BlockAdvancedMachine.isActive(BlockAdvancedMachine.java:203)
at ic2.advancedmachines.BlockAdvancedMachine.a(BlockAdvancedMachine.java:49)
at acr.a(RenderBlocks.java:3237)
at acr.o(RenderBlocks.java:2975)
at acr.b(RenderBlocks.java:136)
at ge.a(WorldRenderer.java:204)
at afv.a(RenderGlobal.java:1385)
at adt.a(EntityRenderer.java:1050)
at adt.b(EntityRenderer.java:848)
at EntityRendererProxy.b(EntityRendererProxy.java:15)
at net.minecraft.client.Minecraft.x(SourceFile:738)
at net.minecraft.client.Minecraft.run(SourceFile:658)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT 93635150 ----------