This turn you should:

1. Build a new rail depot with the Engineer if he has reached the valuable resources. He can build a depot only in the same tile he now occupies. Click on that tile when the Engineer is flashing. A dialog box appears. Click on the “railhead” button.

2. If your miner is idle assign him to mine minerals you have discovered. If you don’t own a miner, it would be a good idea to build one this turn.

3.  Keep your Prospector exploring for more minerals.