Even within one city, London, the trains and platforms cannot agree on basic heights.
Classically we "Mind the gap" between the platform edge and the train on the x or z axis. That comes about because of the curve in the platform but the relative rigidity of the tube carriages. However, the gap we do not mind is that in the y axis.
Train and platform makers: your customers must be the source of your inspiration. Even now, Transport for London only seeks to minimise the step in new trains: "But any new trains introduced as part of the Tube improvement plan should minimise the step and gap between platform and train."
Step-free travel feels like a must, not a "nice to have", in infrastructure design. Allow users to move around. Have a revolution in user thinking.