I commute every day(yes, every day, snow,cold,wet etc) on my Pacific Reach folding bike (about the same size as your Airnimal) and I have no problem what so ever on the Stansted Express, even at crowded hours.
If there is space in the train I don't even bother folding my Reach, I <3 my folding bike, I save £30 on the tube every week, £5 every day on parking space and £30 a month in gym membership. Oh, and I took my car off the road since I just cycle everywhere
That works out at about £3000/year plus the £2000-£3000/year (or more) saved by not owning a car and probably 30 minutes saved each trip on walking from the car park to the train and not queueing to get on the Tube, which adds up to about 10 days per year for a typical commuter of time to spend in bed/at home insead of jammed into a train or rushing to partake of this experience.
Leigh in Redhill calculated a saving of 40 minutes when his work took him to Stratford and he could ride London Bridge-Liverpool Street and get a 7 minute direct train trip instead of a tour of the Tube system. This is clearly a USP for cycling and rail - saving money saving time and it can be delivered without a long time being built and a huge budget for construction.
I've counted at least 2 bikes/minute leaving the main bike exit at Waterloo as an insidious and unrecognised demand. What are your experiences?