Trains with locomotives on both ends

How is a rear locomotive connected to the front locomotive so that the engineer in the front locomotive can control the rear locomotive?

Not sure about the US version but here in the UK it's done via cabling like ETH cables which run throughout the train (there are quite a few trains here that have a loco at one end and what's known as a driving van trailer at the other).

Shane

Further to Shane's has posting above, in Britain the HST class 43 has been operating since the mid 1970s in twin front and rear configuration. They are without doubt the most successful locomotives ever introduced into the UK rail network running in permanent eight car sets. Only now are they being phased out on the Great Western Mainline after over 40 years of service to passengers.

Wonderfully smooth when running at their maximum 125 mph operating speed, I have spent many hours happily working while commuting along with hundreds of thousands of others over the years on the HST 125 class 43s running the Great Western.

Bill

In the US there's a couple different options.

Freight: Long freight trains in the midwest US where long mountain climbs are common commonly use DMU (Distributed Multiple Unit), with extra locomotives in the middle and/or the end of the train. These are typically controlled via Radio Control. In other instances where a helper set is simply added for one climb and then cut back off, they will either also be radio controlled (after the helper's crew connects to the train) or controlled by their own crew, in communication with the head end crew via radio.

Passenger: Typically a true double-ended setup is only used on preconfigured trainsets such as the Acela on the Northeast Corridor and typically only one end is powered at a time anyway, but I'm sure there is a cable link end-to-end to allow either end to become the prime mover if needed. Push-Pull operations, as seen on BNSF's Metra, for example, only have one locomotive per train (the other end is just a cab car, in Metra's case built into the last coach, or in Amtrak's case using NPCUs (Non-Powered Control Units, aka "Cabbage" cars, typically old F40 locomotives with the engine compartment converted to a baggage area) and are setup with control cables running the length of the train via the coaches.