I had mentioned in a past article that I had installed an AirWire system in my battery powered 2-8-0 and was quite happy with the results.
This is a system to control primarily battery powered locomotives, and it has tremendous flexibility along with being "feature rich". 
    The system consists of a Decoder and a Throttle, along with necessary wire and Charging Jack.  The beauty of the system is that you only have to purchase one Throttle, since it can control 8 different frequencies with 1 to 9999 addresses per frequency.  The Throttle is small and easy to hold with a direction control and 9 function buttons. The system itself works extremely well with older 96/97 Phoenix sound systems along with the new 2K2 System and the P5, which was developed to work in the DCC environment. I would also recommend the purchase of the Battery Installation Kit which consists of a pre-wired charging and auxiliary powered jack, and an on/off switch, easy connection terminal strip, and an auto reset fuse for the system.
    AirWire sends you a 35 page manual with their decoder system so that installation is pretty easy.  There is a Toll-Free number available so that you can get any questions answered or problems resolved.
    The decoder installation is relatively easy. It is recommended that it be installed so that there is a good amount of air flow to keep the unit cool during operation.  The antenna must be installed vertically for the best results and the longest range reception.  Two terminals are used to hook up the battery. Two more terminals are used for the sound system that you are using. In this configuration, you eliminate the sound system battery, since the system now draws its power from the batteries that power your locomotive.  Directional headlights are possible with this system, and there is a complete section in the manual that deals with this.  It also gives instructions for using either regular light bulbs or LEDs. 
    The hand held Throttle comes with its own 42 page manual.  It is comfortable to hold, and has an easy to use keypad.  The keypad is used to program the Throttle for various locomotives, and also controls nine functions of the sound system.  With the steam system you get headlight on/off, bell, manual whistle, clank, crossing whistle, coal loading, water fill, volume up, volume down, and blowdown.  For a diesel system you get headlight on/off, bell, manual horn, clank, crossing horn, dynamic brakes, volume up, volume down, and rev up. A tether from a cell phone is recommended so that you do not drop or misplace this unit while operating your locomotive.
    Throttle control is very precise, and the modeler will be pleased with its operation.
T9000 Throttle
Battery Kit