Одиночный высокопольный вагон?? Почему так? /
Single high-floor streetcar?? Why so?

Очень редко. Только тогда, когда вторая машина сломана, и ничего больше не доступна. В противном случае, оно не будет.

That's very rarely. Only if the sidecar is broken, no other available and no low-floor vehicle is available. If a high-floor ST12 is driving on service single, those operations never last so long. It gets a new sidecar then or get changed to a ST13/14. So you should be veeery lucky to get one. It happens something like one time in a half year.