This minecart was removed in Railcraft version 184.108.40.206 (Minecraft 1.6.4) and replaced by the CESU Cart. After Railcraft 9.10, it was re-added as support for IndustrialCraft 2 Classic.
The MFSU Cart is a special type of minecart from Railcraft. This minecart is used to store and transport IndustrialCraft 2 EU via rail. The GUI is the same as the MFSU, only it does not have the Armor section, so to charge armor pieces they will need to be un-equipped before opening the cart. It has the same energy values as the MFSU. Before 8.2, it allows for EU to be transferred at 2,048 EU/tick, and can store 40,000,000 EU. After 9.10, it can transfer EU at 512 EU/tick, and can store 10,000,000 EU.