I bought the train that takes 6 hrs. And it shows 12 hours and won't count down?

Try restarting your game if your timer is not working. Note that the Locomotive takes 1d 6h to complete construction, not 6 hours as you say. If you look under the description 1d looks like "old" but it is actually 01 day, which is 24 hours. So the Locomotive actually takes 24+6=30 hours total.