i remember reading about the concept. maintanence of the cealing flap that held the pressure when not being parted by the, whatever they called it, the part that ran inside the pressure tube to propel the carriage, was something of an issue, that prevented the system from becoming more popular. also i would imagine it actually took more energy to pressurize the system, then to simply propel the carriage more directly by onboard means.

of course the system would appear clean, relatively for those days, at the stations and along the line. but somewhere there would have been a huge boiler, engine, and pump room, or several, somewhere, providing the compressed air for the system.

also the drive tube would have had to have had some controlable method for sequestering portions of it, to enable stopping and stations and starting again (unless there could only be one car on the line, and it was controlled by starting and stopping the pump(s) itself)

Trains would just simply have brakes to stop.

which would increase the load on the power source significantly, and if it were powerful enough, rather then completely stopping them, would instead only flatten the wheels as they were still dragged/pushed along. of course there would be safety valves somewhere to pop, or else the sealing flap, if there were one would act as a relief valve and pop, increasing wear on it and shortening its useful life.