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Trainguard® Lineside  Electronic Unit S21 (LEU S21)

Full interoperability for European railways
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Trainguard LEU S21

The Trainguard LEU S21 (Lineside Electronic Unit) forms the link between the existing fixed signals and the ETCS (European Train Control System) components in the track, the Eurobalises and Euroloops.

The telegrams are transmitted intermittently by means of Trainguard Eurobalises and, if required, semicontinuously by the Trainguard Euroloop. The indicated signal aspect is extracted by the LEU S21 and the associated telegram is selected and continuously transmitted as a serial data stream via a standardized interface to the transparent balise(s) or Euroloop modem. If a train fitted with ETCS equipment traverses the transparent balise(s) or Euroloop, the telegram is transmitted to the train and evaluated by the European Vital Computer (EVC, onboard computer).

Thanks to an identical telegram structure, the LEU S21 can control both Eurobalises and Euroloops; additional data configuration is thus not necessary. A conventional balise/loop antenna is sufficient on the train. 


  • Compliance with all relevant European standards 
  • Modular design
  • Accommodation in concrete equipment houses, switchgear cabinets or relay rooms
  • High-reliability data transmission
  • No maintenance required
  • High level of availability
  • Low purchase and life-cycle costs
  • Low power consumption
Table with technical data

Technical data – Trainguard LEU S21

Current inputs
Number of inputs 
up to 16 inputs configurable
Input range
30 mA to 4 A (15 Hz to 440 Hz)
Flashing detectors
configurable at up to 4 inputs
Voltage inputs
Number of inputs
up to 8 inputs configurable
Input range 
AC 0 Vrms to 181 Vrms (15 Hz to 66 Hz); DC 0 to 181 V
Flashing detectors
configurable at up to 2 inputs
Power supply
Input ranges
AC 88 Vrms to 264 Vrms (15 Hz to 90 Hz); DC 34 V to 177 V
Telegram generator/outputs
2 transparent balises, 2 Euroloops or combined
to 8 outputs with cascading LEU S21
Interface signal C
as per UNISIG SUBSET-036 FFFIS for Eurobalise
Interface signal CL
as per UNISIG SUBSET-044 FFFIS for Euroloop
Number of telegrams / length of telegram
1023 telegrams per output, 341 bits or 1023 bits (selectable)
Reliability (MTBF as per SN 29500)

LEU S21 MS/MT > 31 years 

LEU S21 MC           55 years

Dimensions (l x w x h)
185 x 190 x 286 mm
3,8 kg
Ambient temperature range
–40 °C to +70 °C
IP rating 
IP20 (installation in housing with min. IP54)