One of the cool things about those types of technology is that you can stack them. Something like photovoltaics on top, thermoelectric under that to take advantage of the differential between the heat at the solar cell and something like the ground or the tracks themselves, and underneath all that would be the piezoelectric that you're talking about.

Of course, everything that you add causes some energy loss between each of the systems, but if you've got the money for the hardware, then the sunlight is free, the heat from the solar panels is already a planned energy loss, and the noise/vibrations/movement from the trains are known and accepted losses already. Even though each energy transducer wouldn't be used in its most efficient way, it would still be a net positive.

Hell, make everything easy to disassemble so that it's super-repairable, even though that introduced even more inefficiencies: it's still making use of all the free energy just sitting there and waiting for someone to use it.