The problem is that for high-speed trains you're limited in how small you can make the wheels (that is, even more limited than with other trains – even urban ra

The problem is that for high-speed trains you're limited in how small you can make the wheels (that is, even more limited than with other trains – even urban railways with more modest top speeds need a floor height of approximately 0.9 - 1.0 m for level boarding without having to compromise on the number and location of doors, though with some compromises you can nowadays get down to roughly 0.6 m), and with conventional bogies (and both wheels linked by a fixed axle, which is you how you classically get the wheels to self-steer themselves around curves) it means you need a floor height > 1.1 m for a level interior and level boarding. Conversely for historical reasons however in Germany 0.76 m is the highest platform height that doesn't conflict with the loading gauge of "regular" exceptional loads on freight trains (on a very few selected routes intended for the transport of extra large transformators and similar loads actually anything higher than 0.55 m already causes problems). If you're building a pure high-speed network from scratch without any compatibility constraints (like Japan did with the Shinkansen) you can just choose a platform height of something around 1.1 – 1.25 m and be done with it, but if you're starting with an incrementally upgraded mixed-traffic network as is more common in Europe, you now have a problem… (Talgo sidesteps this problem because of the unconventional bogie design, where the two wheels aren't linked by a fixed axle and steering is assured by the interaction of neighbouring carriages instead, so even with a lower floor height the floor can simply pass between the wheels without necessitating any steps.)