A round-trip flight from Manchester to Geneva would release appropriately 310kg of atmospheric carbon equivalent, plus a train journey on a diesel train.
1700 miles round-trip by electrically-powered high speed rail would be unlikely to release more than about 25kg. It might actually be slightly less than that.
I would happily consider the all-rail alternative, but the complications just consume too much time, cost and energy.
I am actually doing the Manchester-Geneva run in September, as the first leg in a rail trip through the Alps (Covid-19 permitting), returning from Munich.
Manchester-Geneva by rail involves juggling the timetables and pricing structures of Avanti, Eurostar and SNCF.
I haven't found a way of getting the single-leg cost much below £200, and it takes several hours longer (and loses a whole day on the return trip).
Avanti's fares are sky-high, even the CIV ones, which puts prospective northern passengers at a disadvantage compared to London starters (in terms of how far you can get in a day).
The short advance purchase horizons by rail TOCs are also off-putting when you are trying to put a package together at 6-9 months' range.
I can be sure of an air fare/seat and usually hotels up to 11 months in advance, but rail can't think that far ahead - sometimes not even a month.
The return leg from Munich using DB/Eurostar/Avanti would be another £200 and cost an extra hotel night (I don't do sleepers), so another £100.
Thus the two positioning runs (separate from the rail trip in the middle, the object of the exercise) would cost about £500.
I have booked the round trip by air (MAN-GVA, MUC-MAN) for £90 - a no brainer really. Plus £15-20 at each end for local transport.
easyJet, like Ryanair, makes a decent profit on their fare structure (and add-ons), so I'm not sure my air journey is "unsustainable".
Their in-flight service can be endured for a couple of hours without too much discomfort, though I don't like airline security/passport/immigration at Manchester.
I also avoid messing about on the Paris RER and at Brussels Midi, also avoiding multiple passport checks en route (getting worse next year).
I will also be spending money with TfW, SBB, SNCF, Trenitalia, ÖBB and DB on the rail part of my trip, so I'm spreading the benefit.
Greta might not be impressed, but I can't think of a more efficient way to organise a short break to explore the Alps by train.