Hello Haru, The difference has to do with circumstance. In the first sentence, a certain bus did not show up. There is only one bus. In the second sentence, there isn't a single train in operation because the drivers are on strike. All trains won't be running that day. You could say "there are no buses" if there was a city holiday or a strike and all of the buses weren't running and similarly you could say "there was no train" if a particular train didn't show up. The two are not interchangeable and depend entirely on the circumstances. Hope that helps!

It depends on context. If you would only expect to have one, then it is singular. Which i think makes sense since not all the buses can get oneself where, specifically, one wants to go.