It was caused primarily by a signal passed at danger (SPAD) in conjunction with the single-lead junction track layout, where two lines converged into one just beyond the platform end and then diverged again – a layout which is simpler to maintain but is vulnerable in the event of a SPAD.

[1] As a result of the accidents at Purley and Bellgrove, the scope of the inquiry into the Clapham Junction rail crash, headed by Anthony Hidden, QC, was expanded to also cover safety management on the railway on the instruction of Paul Channon MP, the Transport Secretary.

Following repairs to track and signalling equipment, rail services were restored the next day.

An official report,[1] delivered in May 1990, determined that the Milngavie-to-Springburn train had passed the signal at danger, causing a collision with the Springburn-to-Milngavie service.

A fatal accident inquiry was held between 10 and 25 July 1989 by A. C. McKay, the Sheriff of Glasgow and Strathkelvin.

[1] Sheriff McKay declared that the operation of Bellgrove Junction constituted "a defect in the system of working".