Atlanta, having a much smaller transit system, has (or had) what looked like an oversized airline food service cart that was moved by regular passenger service between stations. In K-Mart "Blue Light Special" fashion, it had an amber rotating light on top of a pole connected to the cart. IIRC, there were always two armed employees operating it but otherwise the whole operation seemed rather relaxed. Most stations only had kiosks to vend transit tickets. The ones with manned "Ride Stores" might have had a more secure procedure for moving cash in and out.