I'm pretty sure renewal of life-expired third rail with new third rail doesn't count as an extension and I'm fairly certain this has happened on Merseyrail over the years. If you accept that the tunnels will always be 3rd rail then I don't see why you'd still want to do piecemeal replacements of portions of the surface sections with OHLE.
As has been
posted elsewhere in this forum, even replacements of life-expired DC equipment may favour OHLE.
The new Merserail stock "
could also be ordered with 25 kV 50 Hz capability to allow potential operation to new destinations such as Warrington and Crewe within the next 30 years". Another reason why there would be little incentive to install new 3rd rail.
And it is a faff changing power. It's just yet another thing that can go wrong. The train has to wait longer at the station than it would otherwise. The driver has to remember to lower the pantograph when leaving the OHLE area.
I have experienced that once in the opening year of the Eurostar Service. My Waterloo to Paris service continued coasting at the Dollands Moor changeover point. 22 minutes later was the ominous sign of a man in coveralls walking down the train carrying a large ring binder
Eurostar Technical Manual: Annex 2 - Fault Finding. Turned out that an interlock contact hadn't made when the shoe gear retracted and would not therefore allow the pantograph to be raised. 74 mins late into Paris Nord.
Traction current changeovers are however now commonplace within Europe and often happen at speed on the move. The technical risk would appear to be manageable and acceptable.
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You still haven't said why, despite the regular occurrences of OHLE electrocutions, you believe the Merseyrail system is so much more mortal to people. A couple of articles, one going back over 7 years hardly proves your point. There have been plenty more people who have been run over on the system during this time, so perhaps we should stop the trains running entirely?
The ORR has taken the position that we should not make further installations of a system that causes an unnecessary risk of such fatalities.
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I know experienced posters have raised extra DC track side transformers,but they are offset by ac needing extra rail borne transformers.
They have also raised extra time preparing for track possessions,but that could be managed down & there are benefits,cranes don't have to dodge the overhead line. Power losses,well wet ballast is often piled up against the third rail,leaking vast currents to earth. Take a look next time you are held at a signal in third rail territory.
If NR had a regime for eliminating this sort of third rail earthing the only extra power loss would come from the low voltage of dc,& that could be partially mitigated by low resistance aluminium conductor rail.
Irrespective of the conductor material, losses are proportional to the current. Power is proportional to the product of current and voltage. Transmission systems therefore seek to maximise voltage to as to minimise current.
Those pesky train-borne transformers mean you can keept the supply system at high voltage while meeting the needs of the train. Historically AC control systems on the train were also less lossy than for DC although this is to an extent mitigated by power electronics.
I do get the impression that there seems to be some sort of British passion for 3rd rail systems while other countries have for the most part standardised on overhead systems. High Voltage AC was a sort of holy grail for railways a century ago.
The Bern Lötschberg Simplon cracked this in 1931 with the 15kV low frequency adopted that year as standard for Switzerland (and later for other countries) despite the need for frequency changers.
Why on Earth would we want to perpetuate a dangerous and inefficient system?
