I've traveled quite a bit for business reasons with the Brompton and the Travoy and it works. Never used an airplane with this combo but trains and there is works flawlessly. There are however a couple of issues:
- the Travoy folds well and reasonably quick, but not totally small. So you won't be able to get the trailer together with the Brompton into a bag that fit's airline regulations. With two separate bags it could work.
- The Travoy Bags are not that brilliant for travel. Depending from what you need to transport you may be able to transport the same amount in a big backpack standing the rack and tied to it and the saddle/seat post. Furthermore the alternative bags from Burley are expensive for what they deliver - in my eyes far overpriced, at least here in Europe (can't speak for US prices). And they do not use the full potential of the trailer.
- riding with the Travoy over shorter distances is fine and as a grocery shopper it is a good solution. Longer distances work, too, but due to it's mount on the seat post it "pumps" quite a bit, especially with higher weight on the trailer. Higher speeds downhill leave a bit to be desired as well, to say it politely. Also the hitch on the seat post is in the way when folding/lowering the seat post. The build quality of the Travoy is ok but for my taste there's too much cheap plastic involved and the whole thing is a bit flimsy in itself which possibly has it's part as well in the suboptimal handling when touring. The little peg that leaves it standing upright is a misconstruction in itself - hard to fold due to a harsh bolt that has to be pressed, not too stable (in some situations the trailer will fall over), connected to the trailer with a piece of plastic that broke on the second use of the trailer with me.
Personally I do no longer use the Travoy for that kind of journeys in most cases - I am now using a Bike City Trailer from
Carryfreedom - it seems however to have gone out of production. Here you can see it in a demonstrator video:
It has a couple of downsides and weak areas in terms of build quality of the proprietary bag, the total lack of rain protection and some questionable, impractical aspects of the construction but in general it is a great solution for travel and especially for your intended way. Handling on the bike is simply brilliant even at higher speeds and you can stack additional luggage on top of the standard bag as well.
Carry Freedom do have an alternative in the form factor of the Travoy as well: the
leaf. For travel I'd still prefer a trailer that has it's hitch on the rear axle as these are
far better in terms of weight distribution and handling when on the bike. Their
Y-frame is a classic in that aspect. But regarding folding for airline transport I see it potentially worse than the Travoy. So if I was in the business of searching for a trailer for airline transport and in combination with the Brompton I'd probably go for the
Circe Atlas: Folds very well and small, is mounted to the axle (and thus will have good handling), indefinite luggage options of your personal choice w/o the need for (expensive) proprietary bags and very flexible usage as a consequence. It uses the same lollipop hitch as the Carry Freedom which works brilliantly. I've no personal experience with the Atlas but would not hesitate to buy one if I was in the need.
If you go for one of the above trailers I'd recommend to carry a spare tube with you. They are easy to get hold of but not sponanuously, so it's better to be prepared. I had a couple of blows on the Travoy and w/o a spare tube you are doomed.
The already mentioned
Radical Design Cyclone Chubby is a also a good solution though a very expensive one. Designed for use with the Brompton and with flying in mind. And with bigger wheels than the others. Not as flexible as the Altlas but maybe a bit more comfy in it's dedicated design area. In the end it depends from your needs taste and budget.