電車（でんしゃ）は便利（べんり） ですが 、いつも人（ひと）が多（おお）い。
Ok, so I wrote this one and got it as wrong. The only clue was “but [formal]” . If I understand correctly my answer is also correct and formal, so perhaps one of those “we are looking for other way to say this…” messages would be appropiate?
Hey 
The only clue was “but [formal]” . If I understand correctly my answer is also correct and formal,
You are correct! だが is actually already accepted as the alternate answer 
and returns “we are looking for other way to say this…” .
ですが would be better if it was:
電車は便利 ですが 、いつも人が多いです。
Sometimes the formality gets confusing when you have sentences like this with the formal particle and informal verbs… 