I figured that if the new electric locomotives had a single cab, then an access gangway would exist between the passenger cars and the engine cab so that the rest of the crew could get to it. European locomotive makers only manufacture single cab electrics for higher-speed trains, though (faster than 137 mph).
Amtrak’s due to get new cars from Spanish company CAF soon, with cab cars that allow push-pull operation of the train (engine in the rear pushing the train for half the trips); but that means the engineer is accessible for only one direction of the journey.