Battery Power without using DCC requires the use of an ESC- Electronic Speed Control. ESCs take a servo input and turn it into motor voltage. There are ESCs for ‘brushless’ motors and ‘brushed’ motors. All Large Scale Trains, to my knowledge, use brushed motors.
There are two types of ESC operation. The first is the servo throw goes full left for full stop and full right for full speed. A power relay is used to switch the polarity to the motors to control the direction. The other is the middle range of the servo throw is stop and all the way left is full speed reverse and all the way right is full speed forward.
With the first option, the 8A relay board is tied to one of the widget digital outputs to control direction. The drawback to the relay is that it can, in theory, be switched while the locomotive is under power- this is not a good thing. Since the standard digital outputs on the device are multipurpose, the app does not specifically prevent you from doing this.
One other thing to note is that some ESCs with battery elimination circuits can only supply enough power to run the receiver board. If you want to add servos in addition to the motor control, you will have to supply them using an alternate power source. The Pololu 5v 2.5A step down regulator is a good way to tap 5v power off of your main lipo.