Jis is right. Merely adding a 2nd locomotive to short trains that run slower than 80 mph will compound the problem; there will be twice as many locomotives to fail. The question is why do locomotives fail in the first place and what does it take to reduce the failure rate by 80-90%.
Airlines aren't the same as trains when it comes to passengers held captive by lengthy delays. In theory an airplane can taxi to an unoccupied gate or to a spot on the tarmac where air-stairs can be driven up for unloading passengers. If a train can't move, it can't move -- and it often happens in an inaccessible location. You really don't want passengers dismounting and scrambling around on ballast and ties, dealing with the detritus that is often found along the tracks.. not to mention trestles, etc. The real question is one of design: how to ensure that HEP is available 99.99% of the time, even if the prime mover of the locomotive goes down, even if a traction motor freezes up an axle, etc.