The report is clear on this.The 1962 Ministry of Transport official report into the electrical failures and explosions on the new high voltage AC lines in both East London and Glasgow has definitive maps on the last page of the detail of what was at 6.25kV and what was at 25kV. As the changeover equipment from one to another was at the centre of the investigation, and as the maps show detail of all the voltages of the various lines and the feeder stations (four needed for Shenfield to Southend; just one needed for all of Colchester to Clacton/Walton) and other aspects, I think they are unlikely to be wrong, as at 1962. The changeover to 25kV is shown north of Shenfield on the GEML. This section to Chelmsford had indeed initially been DC, but only had an hourly local service in DC days. Here it is, last page:
Paragraph 21 on page 6 includes table 1 which shows Shenfield-Chelmsford as 25kV and converted from 1500 volts DC.
Paragraph 42 on page 9 says:
Following the adoption of the high voltage A.C. system as standard for British Railways, it was decided to convert this overhead D.C. system to A.C. in readiness for the opening of the new A.C. services in November, 1960. It was considered that the clearances already provided for 1,500 volts would be suitable for 6.25 kV A.C., and consequently the major part of the system (all except the Shenfield-Chelmsford line) was converted to this voltage. The Shenfield-Chelmsford line was converted to 25 kV on 20th March, 1961.
Although the report is dated 17 March 1962, it was commissioned on 20 December 1960, and therefore mainly relates to events that took place before that date. Trains changing over between 6.25kV and 25kV at Shenfield wasn't happening in November and December 1960 because the 25kV electric service to Chelmsford did not start until 20 March 1961. There were issues with voltage changeovers at Shenfield and these are covered in Section XVI of the report starting on page 55. Paragraph 262 says:
262. In March, 1961, the Shenfield-Chelmsford extension was electrified at 25 kV and no troubles of any consequence were experienced until 29th May when an air-blast circuit-breaker failed to open while the train was passing through the Shefield voltage changeover section, and this caused a serious failure of the transformer. Several other such failures have occurred at the same place and these are described in the paragraphs that follow.
