The Victorian trainshed - an elegant and effective solution to keeping people dry in a smoky environment.
Third rail electrification - relatively easy to install, robust and effective at what it does.
The 1st class config Mk3. To this day, no better travelling environment.
EPB's. Basic, yet comfortable and effective at getting thousands to work and back every day.
The L&SWR, like all railway companies, developed over time, but developed into the perfect tree like network of trunk, route and branch, aided by the many grade separated junctions installed.
Class 158's/156's - excellent workhorses of the Regional railway and pleasant to travel in.
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The British Rail corporate identity of the 1960s, not least the double arrow and the Rail Alphabet, the first of which lives on everywhere. The second would still be with us everywhere if someone had properly thought through privatisation in the 1990s.
Does that include Trojan and Bournemouth Blue moquette ? I think it should.
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I should also add, the Southern Railway/Region. Large scale electrification, fixed formation trains, clock face timetables. They must have been doing something right because the whole railway has copied them.