Town hall is responsible for the amount of travellers that can visit your town at the same time. At first your town can accept only 9 travellers, but with each upgrade 3 more travellers will be able to visit the town. If your town is full, the train won’t bring any new travellers (or will bring less than expected). Upgrades require trowels, hand planes and paint brushes.
Also you can use the Town Hall in order to send travellers to different buildings.