New transit systems will generally have to comply with the HMRI requirements as posted by greybeard33. The national rail network has "grandfather rights" and there would be major challenges with meeting this requirement especially at platform where freight trains also pass.
Expressed in old money, maximum modern clearances translate to approximately 2" vertical step and 3" horizontal gap. I assume Metrolink and T&W were working to similar standards before they were enshrined in these latest regulations.
Although they'll achieve that on the new Crossrail central core and Abbey Wood branch with all new platforms, that project will be unlikely to be able to achieve anything close to the standard at the existing stations also served on the GWML and GEML, despite new trains and a significant increase in service frequency, especially on the GWML relief lines. I can understand that from the freight and other traffic compatibility reasons you cite and the sheer cost implications of building up and resurfacing all those platforms. I wonder how long these heavy rail grandfather rights can persist however, and TfL are arguably in an even more difficult position than NR regarding horrendous gaps at some of their extremely busy tube stations. Raised platforms and extending step boards on the vehicle seem to be a very good solution for a number of applications:
- Classic compatible trains at UIC clearance platforms on HS2.
- New heavy rail stock AND (perhaps narrower) high floor tram-trains on the general classic rail network at platforms where the edge has to be set back a little further than ideal for dynamic clearance to passing trains at speed.
- Sharply curved platforms where vehicle end and centre throw of stopping and passing traffic demands a large gap (particularly applicable for TfL tube).
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