This has got quite a distance from Merseyrail rolling stock, but I think there are some relevant points.
Conversion of existing track from third-rail to overhead might make sense if:
- The third-rail equipment is life-expired and:
- There is a need for more power than third-rail can reasonably supply (either for freight or for lots of 200m+ services) or
- There is a need or desire to run trains above 100mph
In that case, replacing with OHLE will still be more expensive than renewing the 3RE, but much less so than replacing 3RE that has significant remaining life.
For Merseyrail, the 3RE is a long way from end-of-life, and there's no freight nor any prospect of any, and the platforms aren't long enough to run 200m+ trains on any part of the network - and the PSRs are well below 100mph, so any desire to run faster trains can be met in the foreseeable future with faster third-rail trains.
The only argument for converting parts of the network would be if there was a desire to run overhead-only electric trains into some Merseyrail stations without needing to equip those trains with dual-mode power supplies, and I can't see many places that would be interesting, as overhead-only trains would never be able to run into the Merseyrail tunnels (so only into outer parts of the network, not into Liverpool). The only even distant potential I could see would be possibly extending a London-Chester train to Hooton if Crewe-Chester gets electrified at some future date.
On the other hand, there are plenty of places that Merseyrail trains could be extended onto track that is or could be electrified with overhead power, so having the ability to run onto AC overhead makes a lot of sense.
For instance:
- Bidston-Wrexham, if ever electrified, is likely to be AC overhead.
- Extending Northern Line beyond Ormskirk/Kirby (though I think that should be third rail as far as Wigan Wallgate)
- Ellesmere Port-Helsby if overhead is safer through the refinery than third-rail
- If the CLC route from South Parkway to Deansgate Junction is electrified, then extending Merseyrail trains from Hunts Cross into Warrington would require dual-power
- A curve at South Parkway to allow WCML trains to run into Northern Line might allow Merseyrail to run via Halton Curve to Chester.