Taxi from TaiYuan trainstation to Pingyao old city.

It is a little further than 100km between them, so it will be pricey to take a taxi for the journey. Also it is that late, they may charge you around CNY1,000. If you don't mind, you can transfer to another train to PY first and then take a taxi to the old town. (I remember there is a train leaving around 23:35 and you'd better prebook the train ticket.)

If you miss the train, you are advised to stay in Taiyuan for a night and head for your destination on the next morning.