In 1912 were trains that had a narrow passage running along the compartments common?

Yes. This pattern was common in sleeper cars and private compartment cars. An isle down the center of the car was fine in coach cars with conventional chair seating, or lounge cars too. But a center isle would not work in a car with compartments or sleeping berths because there wouldn't be enough room for such compartments on either side of the isle.

Which country are you asking about?

In the UK this type of carriage was fairly rare at that time. Most carriages were just divided up into compartments with a door either side to get down onto the platform.

no/half were open cars/they were pretty primitive in those days/no steam powered brakes/brakemen on all cars with manual brakes/little hairy to ride for a great distance