Nothing is redundant on a bike, if anything it is the other way around, e.g. the brakes are the wheels, the frame is the suspension, the gear mechanisms are also there to keep the chain on, the passenger is the power source. Nothing can be taken away.
Bikes also pioneer high tech lightweight materials, only recently have 'bicycle grade' aluminium and carbon fibre been used on those boxy car things.
It is a whole different world of 'body on frame' engineering. I sometimes wonder what a train would look like if it was designed from the ground up by a bicycle designer with a Lotus engineer - 'add lightness'.
Some modern 'Tesla truck' tech could be thrown in too.
Imagine having half the amount of wheels, all of them actively powered and actively steered so no screeching on corners. The seats could be 'bus stop grade' and the monocoque being some extruded tube with the hand rails and luggage racks being structural elements, like a rally car roll cage.
The idea of Brunel's wide gauge was to have the wheels at the side of the train rather than have the train sit above the tracks. Imagine if you had independent suspension and stepped a metre down into the train for some super low centre of gravity. Or if that extra space below the train was a massive parcel area so goods were shifted on/off trains with some standard pallets that just slotted in, aircraft style. Think how much traffic could be shifted off the roads particularly if you ran the trains in mixed freight mode through the night.
Rollercoaster style tracks, particularly in tube lines could be used to slow down trains at stations and speed them up afterwards.
As for the doors, these could be integrated into the compartment ends, so rather than there being the normal interconnect between compartments the doors would slide back/fore to keep the train enclosed on corners without the usual rubbery bendy bit needed.
Add airbags and seatbelts, windows that open, a bit of wifi and you could have comfort and actual passenger safety.