ERTMS

European Traffic Management System "ERTMS" is a traffic management system used in Europe. It's first use in the UK was on Cambrian Line in 2012. There are few levels.

Levels
Level 1 - Provides Automatic Train Protection (ATP) functionality overlaid onto conventional lineside signalling with continuous train detection. Infrastructure data is stored in 'lineside electronic units' (LEUs) which are distributed along the lineside. Movement authorities are transmitted to passing trains by transponders, known as 'balises', mounted in the 'four-foot'. ERTMS Level 1 uses controlled balises, capable of transmitting variable data according to signal aspect. They receive their information via the LEUs. Trains not fitted with ERTMS trainborne equipment can continue to operate in accordance with the lineside signals and under the protection of AWS and TPWS. This is also the fall-back mode of operation if ERTMS should fail. Level 1 delivers interoperability and improves safety but the intermittent nature of the ATP supervision leads to a small reduction in line capacity.

Level 2 - Continuously transmits movement authorities to the train via a GSM-R radio network. The infrastructure data is held centrally at the control centre. Balises are installed only as position markers, transmitting fixed data to initialise and occasionally recalibrate the train's odometer. Continuous train detection is provided by track circuits or axle counters. Although ERTMS Level 2 is designed as a standalone cab signalling system, it can be overlaid onto conventional lineside signalling so that trains not fitted with ERTMS trainborne equipment may still run on the line.

Level 3 - Does not require continuous train detection. The train determines its position from fixed balises (as per Level 2) and reports this information to the control centre via GSM-R radio. The control centre determines track section occupancy from position reports received from trains and continuously transmits movement authorities to trains via the GSM-R. Since the train continuously monitors its own position, Level 3 can support moving block operation, which provides the best possible line capacity. In the absence of conventional train detection, the trainborne equipment must continually check that the train is complete. Only trains fitted with ERTMS trainborne equipment can run on a Level 3 fitted railway.

Cambrian Line
The Cambrian Line uses Level 2 ERTMS. It had been signalled in January 2012 to replace the old RETB system.