I can't see the logic in this argument; after an initial charge of the system whist stationary, regerative braking is just the process of converting kinetic energy to stored energy, which can then be used to re-accelerate the train again to full motion, converting the stored energy back again to kinetic. This just means that the energy used to propel the train is somewhat conserved (subject to energy cycling and aerodynamic efficiencies), during operation of the train.
The solar panels then in theory only need to supply the energy needed to top up the energy lost from the system due to cycling inefficiency
All of this is independent of the carbon neutrality of the system.