Originally Posted by jsloan

A United to United transfer -- and, actually, many Star Alliance to United transfer -- will result in checked luggage being sent to the midfield terminal. There's no place to walk.


Try multi-city search.


At IAD in particular, you get on different moon buggies after deplaning -- one for passengers terminating at IAD (to the main terminal), and one for passengers connecting to another destination (to the midfield terminal). That's the rub -- if you check bags, you'd have to go retrieve them at the midfield terminal, and then you're effectively trapped.

As a reminder: if you choose to do this, UA has no obligation to send you to IAD. If there is a schedule change, or IRROPS on the day of travel, UA may choose to put you on another flight to YYZ. This would be particularly plausible on an ex-EU flight, because UA would be liable for EC.264 compensation if you did not arrive at YYZ within 4 hours of your scheduled time, so if they've got a delay and can put you onto an AC nonstop or something, they absolutely will.