Wasn't there a lot on consternation back when this was all very new and very much an unknown? ISTR reading that it was regarded that a (now known to not be required) excessively large distance was required to prevent "incidents" between the steam and the wires at 25kV AC. This is what lead to the tracks being lowered, neutral sections under bridges and 6.25 kV AC being used in constrained locations when the GEML was converted from 1500v DC to 25kV AC.
We now know this isn't required so the voltage was upped to the standard 25kV, but still.
The GEML was electrified to the (then) standard of 1500VDC. This mandated a static clearance of 4 inches from conductor to grounded structures and vehicles (e.g. roofs, funnels etc.). The conversion to 25kVac OLE mandated a static clearance of 11 inches to grounded parts. For locations where that clearance would have been too disruptive or expensive to maintain, a nominal voltage of 6.25kV* was allowed using the existing 4 inch installation clearances. In the case of the GEML, this meant that the physical OLE from Liverpool St right out to the Mountnessing 25kV switchover point (on the main line just past Shenfield), could be used unchanged, including the original 1500VDC insulators. By the time that the conversion was undertaken, the line to Southend Victoria was also wired with 1500VDC, so that was destined to be the reduced ac voltage whereas the main line from Mountnessing out to Clacton and Walton would be at the full 25kV. There were similar arrangements south of Manchester.
The method of providing a 25/6.25kV capability in locos and EMUs was to have a main transformer with four equal separate primary windings. By means of switchgear, these windings could be connected in series for 25kV operation and in parallel for 6.25kV areas. The switchgear was under the control of trackside baliases, and set the switchgear for the appropriate supply voltage at the changeover point.
This dual voltage arrangement could have survived to today except for two issues:
1) some of the EMUs had a design shortfall that meant occasionally, they would not satifactorially revert to the full serial 25kV configuration when entering a high voltage section. This applied four times the nominal voltage across the transformer primary causing 16 times the normal power dissipation in the transformer, with spectacular catastrophic failures, usually involving a fire that destroyed the motor car.
2) the number of trains running on these lines and their lower power requirements in the early days was not a siginificant problem. It is likely that current power demands would have required an increase in the number of feed points at 6.25kV. The GEML did however have the benefit of much heavier OLE conductors and catenaries, originally required for the lower DC voltage.
At some point in the mid '60s, tests were performed (ISTR at Crewe) where a steam loco was parked under the live conductor which was progressively lowered until there was a flashover to its highest point. This determined the smallest safe clearance for 25kV which I think was about 6.5 inches.
Following the redefiniton of the minimum distance, it meant that the lines with 6.25kV could be upgraded to 25kV, albeit with some structural modifications and certainly for the GEML, the replacement of the original 1500V simple insulators with compact multi-disc versions suitable for 25kV. At the same time, many of the original feed points were disconnected and the remainder upgraded to 25kV.
* The original report in the '50s recommending the lower voltage for reduced clearance stipulated 6.5kV following the trial on the Lancaster, Morecambe and Heysham line. This was later reduced to 6.25kV (25kV/4) to allow the use of four-primary transformers in the trains (see above).
