Which is the longest railway platform?

Why is the railway platform height ("train floor height") so diverse around the world and between train types (metro vs. long-distance trains)?

• See also: https://en.wikipedia.org/wiki/Railway_platform_height

• Answer:

  The whole point of a metro is speed of loading/unloading.   Imagine trying to get everyone on and off the train quickly when they have to climb up and down steps to get in and out.   Having said that I'm sometimes surprised by how some suburban railways have a mixture of low and train floor height platforms. In any modern, new build railway designers have to take into account "Persons of Reduced Mobility" rules.   You have to build your system so that anyone - ANYONE - can use it.   That means you have to build things like signs that deaf people can read, make announcements that blind people can hear and make the platform the same height as the train floor so people in wheelchairs can roll on and off. None of that was applicable in days gone by when the railways were originally built.   To start with there were no standards.   The so called "standard gauge" (4 ft 8 1/2 in) wasn't standard together.   Railways were built with all sorts of track gauges and with all sorts of heights.   As things coalesced and standards began to appear it was deemed too expensive to adjust all the various station platforms.   In some places the platforms were already a compromise.   Take Hammersmith on the London Underground; it is served by the District Line (main line sized trains) and the Piccadilly Line (so called "tube" trains - much lower.   The platform is arranged so you can step out of one train, cross the platform and in to the other.   You step down from the District on to the platform and down from the platform into the Piccadilly.   Make the platform the same height as either train and the step would be enormous.