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I was on a double-decker RE to Friedrichshafen, in September, and I didn't find it particularly comfortable (cramped, I would agree) but they're fine for shorter journeys.
Shorter journeys, yes, but they're also increasingly being used for ICs, not only for Regionalbahnen or Regionalexpresse. There's also the issue with the €9/49/whatever next ticket that many more people are now taking long trips by non-IC or ICE trains, so there are definitely more longer journeys being made on them in this way, and not just in last year's cheap holiday frenzy mode.
Re voltage, I think new trains can be configured to operate at different ratings.
Yes, the ICE between Frankfurt and Bruxelles (Midi) does that. It changes voltage somewhere around Aachen. Once recently that facility was broken and because they've taken away all the slower trains between Aachen and Bruxelles it all became quite difficult.
I'm at work... ironically.
I was on a double-decker RE to Friedrichshafen, in September, and I didn't find it particularly comfortable (cramped, I would agree) but they're fine for shorter journeys.
Re voltage, I think new trains can be configured to operate at different ratings.
In terms of track gauge, most of Europe uses Standard which is 1435mm (4ft). Russia/former Soviet states uses 5ft (~1520mm), Ireland 1600mm and Iberia a whopping 1668mm.