Actually, CO/UA flies a lot of mainline equipment from EWR-CLT- anything from the 737-500 to 737-800, regularly. Most of my CLT-EWR flights within the past two years on CO metal have been on 737s, and often the cabin has been quite empty, especially on early morning departures. More recently, UA has introduced the E175 on certain EWR/CLT departures as well. I recall the fares being quite low, perhaps due to the amount of extra lift. Last two times I flew CLT-EWR in the previous few months (not connecting through), I paid $269 rt on CO, and $156 rt on US- Friday outbound and Monday inbound. I suppose it depends on when you fly and when you book.

Along these lines, CO/UA has now significantly increased the lift on the IAH runs with several departures on 737s. When I have my choice, I always try to use CO/UA when flying to/from EWR as other posters have already mentioned, the US terminal A is one of the worst I've seen anywhere, with zero FF amenities. Even the elite security line is often longer than the regular one.