Yellow Fleet
What is it
The Yellow Fleet is Network Rail’s dedicated fleet of infrastructure monitoring trains and vehicles used to measure, record and image the condition of Britain’s mainline railway. These vehicles are painted in high-visibility yellow and carry specialist instrumentation for track geometry, rail flaw detection, overhead line and lineside inspection.
Why it matters
The fleet enables condition-based maintenance by detecting emerging defects in track, overhead line and structures before they become safety risks or cause service-affecting failures. Running largely in traffic paths between timetabled services, it reduces the need for disruptive possessions and manual walking inspections, saving millions of pounds each year.
Which vehicles
The Yellow Fleet spans slow, medium and high-speed monitoring vehicles, including the New Measurement Train (NMT), ultrasonic test units (UTUs), structure gauging and track recording trains, overhead line inspection vehicles and Plain Line Pattern Recognition (PLPR) units. It also includes converted coaches and MPVs fitted with geometry and video systems for switches and crossings, radio survey vehicles and other departmental vehicles used for visual and structural inspection.
Who owns and runs it
Network Rail owns the core infrastructure monitoring trains and manages them through its data collection and fleet teams. Operation and maintenance are delivered by a mix of in-house staff and specialist suppliers, with on-train technicians responsible for running the monitoring systems and handling the recorded data.
When was it introduced
The Yellow Fleet expanded significantly after Network Rail’s creation in the early 2000s, with the NMT entering service in the mid-2000s as the flagship high-speed measurement train. Having significantly improved UK rail infrastructure monitoring for more than two decades, the Yellow Fleet required updating by the mid-2020s, when Network Rail launched a Rail Infrastructure Monitoring Service Replacement procurement to replace the existing yellow fleet methods over the coming years.
How it works
On board, the trains capture data across all rail disciplines, from track geometry systems (including conductor rail geometry) to laser scanners, high-resolution video (including FFV and PLPR), ultrasonic rail-flaw detection, ground-penetrating radar, LiDAR, overhead line laser/camera systems and radio survey equipment. These sensors stream synchronised, GPS-located data into processing systems which automatically calculate geometry parameters, detect defects, generate exception reports and deliver image and measurement datasets into Network Rail analytics platforms for maintenance planning.