MU means "Multiple Unit". It is a process whereby twin locos are coupled to each other in such a way that they operate as a single loco and co-ordinate with each other. This technology is used in WAM, WAG, WDM and WDG series locos. WAP-4 locos cannot be "MU'ed" i.e. twin WAP-4 cannot run together live on a single train, since they were not enabled to do so. Now with MU technology, this twin WAP-4 operation on single train will be possible. That would be 10000 hp!!