Basically the latter: key factors are minimum curve radius, distance of the two tracks and distance from buildings/walls, switches. But other requirements are: stronger and more tense catenary, special signalling and train control system, and a number of safety measures (like sensors for cars falling off bridges).

To bolster you even further, here is a picture of what someone referred to upthread, a TGV pulled by a diesel on the last leg of the Paris--Les-Sables-d'Olonne journey along a really really conventional track:

_{*Lunatic*, n.
One whose delusions are out of fashion.}