Pocket Trains [Android]
Set up train routes that travel in a straight line towards one connecting city. For example, use Paris as a connecting city at the start of the game. Build five or six lines that all end at Paris. Then, upgrade the Paris station to hold as much cargo as possible.
Spend Bux to build trains, even if they are not needed. You can sell those trains for a lot of money.
Watch the game while your trips are in progress to have a chance of receiving gifts, which include Bux. Watch the ad videos to earn free Bux. During levels 3 and 4, there are lots of opportunities to make trips to earn Bux.