Other than the walkthrough element, that sounds pretty much like Hitachi's original proposal for the Intercity Express programme, which envisaged a pantograph fitted seated driving trailer at one end of the train and a diesel power car at the other, or CRSE's proposed Polaris trains, which would have had a bi-mode power car at one or each end of the train. A few images shown below:
The dedicated diesel power car element of the Intercity Express proposal was dropped when it was recognised that the power cars could be rendered obsolete by further electrification within the planned lifespan of the train, and removable underfloor engine rafts beneath a standard design of passenger vehicle offered more flexibility along with the benefits of distributed traction and built-in redundancy in the case of an engine failure.
(Apologies for the off topic IEP discussion
, but it seemed relevant to the point being made)