I am building a train layout and I am running some of my track through a tunnel. I would like to know if I could hook up a 9v train motor at the opposite end of my train. The front will use Power Functions for the motor. Will this work together in case my IR receiver loses communication to the controller and at the same time run at the same exact speed? Is there an alternative?
I'm not so sure using 9V will be usefull ?
My son has a PF cargo train in his room, when he turns it on, it goes, ok.
but even if my son goes downstair to get a cookie while he still has the PF emitter in his hand, then train still runs !
According to me, PF infrared link is used only when you wish to change speed setting, so if you don't plan to slow down your train in the tunnel, you are ok.
One option is to only use the 9V train motor, but power it most of the time with the PF battery back. This will require some customization of the motor, and probably some cutting/soldering of the wires, but this would work. (See: Can 10153 9V train motor be powered by the Power Functions Battery pack?)
Another solution is to add a second battery pack in parallel to the first, and only turn it one when the train is in the tunnel. You can mechanically (and automatically) turn it one when the train enters the tunnel, and then turn it off when the train leaves the tunnel. A small switch under the train (using something like two of these to make a switch that can be "flicked" on and off).
If you don't connect the two subsystems electrically, then it is quite possible to do so. The Power Functions motor will be on when it can get a signal and the 9V train motor will be on when the train runs over the metal rails. The metal rails will need to have external power attached, and you have to adjust the speed there separately. The mechanical resistance of the unpowered motor will probably have undesired effects though.
If you can't make a seamless transition from unpowered rails to powered ones, then you might look into building a more complicated setup where a PF remore receiver "sets" some kind of mechanical variable that controls the on-board motor output. This way even if you lose PF reception, the train will still run at the speed you set last.
If your train is longer than the tunnel, you could also install two PF receviers, one in the front and another in the rear. This way one of them will always be in range.
You could also consider using an SBrick. It controls PF units via Bluetooth, so no line-of-sight is required.