Radlett Strategic Rail Freight Interchange Engineering Stages 3&4 Signalling Design

DigiSig Rail Group were instructed by BWB Consulting to undertake the Engineering Stages 3 & 4 Signalling Design for the Rail interface required for the new Rail Freight Interchange at Radlett Aerodrome. The Radlett site was originally opened as an airfield in 1930 where it was used for manufacturing and building hundreds of military and civilian aircrafts before the demise of the factory in 1970. The strategic building plans put forward by architects to transform the old Radlett Aerodrome was that its main purpose should be structured around a sustainable movement strategy which would include a new rail freight interchange.

The new Rail Freight Interchange also required an intermodal terminal which would be serviced by a new rail connection to the Midland Main Line (MML) corridor, at an approximate chainage of 17m 20ch near St Albans. At this chainage the MML comprised of a four-track electrified railway, on embankment and of a straight alignment. The MML is within the East Midlands route, with the project area affecting the lines associated with the Engineers Line Reference (ELR).

The new connection from the SRFI to the MML actually passes beneath the MML via a new intersection bridge which created a grade separated crossing before connection to the existing Up and Down Slowlines. Line speeds in the area for passenger trains is 90mph and 60mph for freight.

The proposed site of the new rail freight connection to the MML was located between two interlockings, St Albans and Radlett interlockings, which consist of Westpac Mk 4A geographical interlockings, dating from 1979. Alterations were carried out to Radlett Junction, but no alterations were required for St Albans Interlocking.

Site Location Plans

Our Role

Radlett Depot Junction

The project is to provide an island relay interlocking, to control the Up and Down Slow lines only. To simplify the interface to Westpac recoveries only, the island interlocking would include the control for Radlett Private Siding, and integrate with the existing interlockings at the following existing auto sections:

  • Up Slow – WH264 to WH256 (exclusive)
  • Down Slow – WH263 to WH269 (exclusive)


The team at DigiSig Rail were engaged to undertake the following activities to facilitate the project:

  • Create Base Plan and Update to Site Correlation.
  • Development of a Scheme Plan.
  • Design Log.
  • TPWS Calculations.
  • SSpaM Assessments calculations were undertaken during the option selection process to determine the optimum position for signals protecting retaining adequate spacing.
  • Aspect Sequence Charts.
  • Signalling Design Specification.
  • Interlocking Control Requirements Specification.
  • Risk Assessments.
  • IDC, MSRP and HAZID engagement and support.
  • Location Area Plan.
  • Depot Control Option Selection Report Standards.


Network Rail Company Standards & Railway Group Standards were applicable to this project along side Signalling Design Standards:

  • Rail Safety and Standards Board: NR/L2/SIG/11201
  • Signalling Design Handbook & NR/L2/SIG/30009 Signalling Principles Handbook.
You can download a copy of our case study by clicking here

Simon Ward BEng(Hons) CEng MIET Technical Director - Rail Systems & QHSE Compliance BWB Consulting Limited

No complaints raised and no Health and Safety issues. Overall good engagement with the team as required for Engineering Stage 3. Also thanks for the successful delivery of Engineering Stage 4 and continued support on this project.

Project Information

  • Project Title: Radlett Rail Strategic Freight Interchange Engineering Stages 3&4 Signalling Design
  • Client: BWB Consulting Ltd
  • Date: December 23
  • Category: Signalling Design
  • Status : Completed