Even if the train has departed the station by the time you're putting the bike in a rack, the person, bike and rack are not in motion relative to each other so your car analogy is slightly ridiculous. It's more like can you put your bike on a rack in your garage next to other bikes. Unlike a car rack there is also generally much less lifting of the bike as you rotate it on the rear wheel then there is only a small amount of lifting required to hook the front wheel onto the rack, where a car rack requires you to effectively overhead press a bike onto a car roof which is increasingly over 1.5m high.The most obvious difference is that you rarely have to insert a bicycle onto a roof rack between two others belonging to strangers while the car is in motion. Apart from that…
Hanger racks are useful as they increase the number of bikes which can be transported allow the space underneath (if empty) or the side (if partially full) to be used for luggage, skis, kayaks etc depending on the season and demand. A decent sized luggage (including bikes) area is something which should be in spec for units operating on the Highland, Aberdeen-Inverness, West Highland, far North and Kyle lines, but a flexible one, probably about the size of the first class compartment which was originally on a 2 car 158.