http://www.therailengineer.com/2013/04/12/being-selective/
The Siemens Desiro trains of Classes 444 and 450 have a modern Train Management System (TMS) that services the whole unit and thus a reader has only been fitted to every cab vehicle. The reader picks up data from the beacon and enters this into the TMS via an RS485 communication port. All readers are active but only one is needed to acquire the platform information. The TMS will then know where the train is, if it is due to stop at the station, which side the platform will be and which doors to release if it is a short platform. The stopping accuracy by the driver is based upon ± 2 metres from the platform marker point. A total of 344 readers are being supplied for the Desiro fleet, these being fitted at the Northam depot near Southampton.
The Class 458 Juniper trains from Alstom are an older design with only a limited TMS which is already full and would be expensive to upgrade. Thus it has been necessary to fit every carriage with a reader, which will include the additional carriages to be inserted to make the 5 car unit. The operation will be different; each reader will connect to that carriage door control and every carriage that passes over the entry beacon will have the door opening enabled.