I have found no correct answer to this problem. Unlined trenchcoats provide rain cover but are not warm. Anything warm that fits over a suit will be bulky. Sometimes I just hope I won't be outside long and wear a heavy sport coat and carry an extra sweater or two.

 

My topcoat choice depends upon where I am going, the expected weather when I get there, and the purpose of the trip. Meeting investment bankers in Zurich during the winter is different that meeting San Francisco mobile app developers in October.

My "general purpose" travel topcoat is a dark blue, knee-length trench coat with a zip-out wool lining (body and sleeves). I usually leave the lining in my suitcase and wear/carry the trench on board. The removable lining gives you additional flexibility to be comfortable in a range of temperatures. It's warm enough for all but the coldest weather, water-resistant and can be dressy with a suit or casual with jeans and a sport coat. It was the perfect choice for a month-long business meeting in Beijing during the winter. I bought it at Macy's about 10 years ago for around $250 and it has served me well.

I like traditional beige/tan trenches, but they show too much dirt to be good travel coats. I also have a elegant Gianfranco Ferre charcoal wool top coat which is my "formal" business and out-to-the-opera top coat. It's a beautiful coat, but not for air travel, especially to rainy or snowy destinations. When I travel (especially internationally), I take a nice (but not too nice) trench-style top coat. Through a sweater vest in your suitcase for extra warmth and you will be good to go.

 

Down compresses quite well and is warm. There is a limit to how formal an appearance one can achieve, but not all look like ski wear.

 

My "general purpose" travel topcoat is a dark blue, knee-length trench coat with a zip-out wool lining (body and sleeves). I usually leave the lining in my suitcase and wear/carry the trench on board. The removable lining gives you additional flexibility to be comfortable in a range of temperatures. It's warm enough for all but the coldest weather, water-resistant and can be dressy with a suit or casual with jeans and a sport coat. It was the perfect choice for a month-long business meeting in Beijing during the winter. I bought it at Macy's about 10 years ago for around $250 and it has served me well.

I think this is very good advice, though I must say I do embrace the beige.

If you must take a wool coat wear a closer fitting topcoat as it will weigh much less that a great/larger coat and will thus make a difference when stowing or walking any distance, particularly in an airport.

 

I am amazed you got by in Beijing winter with just a trenchcoat. When I spent a week there a few winters ago it was about 10 F (-12C) and windy. There's no way a mere trench, lined or not would have been adequate for me, even for brief ten or fifteen minute walks. So I guess that is relevant information for the OP. How comfortable are you in the cold?