A rail system for supporting a patient gantry including a fixed rail assembly mounted on a support surface and a service rail assembly releasably securable to the fixed rail assembly. Each rail assembly includes a bar and a top rail. An alignment device is provided for aligning the top rails of each assembly and is configured so that the rails cannot be interchanged. A lever is provided for positively securing the service rail assembly to either the fixed rail assembly or in a first receptor defined by a mounting block secured to a bearing block and on which is mounted the patient gantry. A toggle clamp includes a pin selectively received in a locating hole defined in the fixed rail assembly. If the toggle clamp has not been engaged and the lever does not engage the first receptor, the lever is received by a second receptor to prevent derailment.