There are times when you need to connect two devices without running a redstone trail between them. Wireless redstone is available, and has the benefit of a "remote" that you can carry, but it can be quite demanding to craft.

The Railcraft Controller and reciever circuits are relatively easy to craft, most established players will be able to collect everything except possibly the slimeballs.

Each receiver needs to be "paired" with one controller. You'll want to craft an extra receiver circuit, in order to make a "Signal tuner". If you want to control switch tracks then you'll need to craft some receiver circuits into switch motors, otherwise you'll craft "Signal Receiver Box". The Controller circuits need to be made into the "Signal Controller Box"

Most guides refer to using these for block signalling, which is complicated and so I'm going to skip over it for now and assume you just want to make a row of levers to control your switch tracks and other redstone functions.

The Signal Controller Box can be operated by a redstone signal. On its own it defaults to a "green" condition, changing to "red" on receiving a redstone signal. There is one peculiarity that may confuse you: The box will not react to a directly adjacent lever or torch. It will react to active redstone dust leading to it, it will react to a redstone repeater and it used to react to the signal from a Railcraft detector box though this may have stopped working in which case you'll need a redstone dust between the detector and controller.

If you have a wall with a row of levers you can place a row of repeaters behind the wall operating a row of controller boxes.

At the recieving end:

Switch track:
If you want to control a switch track then place your switch motor and use the Signal Tuner to pair it with a controller, controller first then motor. You can configure the motor to determine which states the motor operates on. By default it operates on "Red" meaning it operates when your lever is down, and you need a crowbar to access the GUI to change it.

Other redstone function:
If you want to control something else then place a Signal Receiver Box where you want the signal and use the Signal Tuner to pair it with a controller, controller first then reciever. By default it will output redstone on Green, making it inverted. I recomend changing the receiver to output on red, so its "Red=Redstone".

You can pass on the signal by placing a further controller next to the receiver but there are special rules for this, the redstone settings are ignored and it is the signal colour that is passed on.

RANGE:

There doesn't seem to be a range limit for controller-receiver pairings but both chunks must be loaded in order for them to work correctly. On Crazy Fools that sets a practical limit of about 80 I think. I'm not sure what the chunk load radius is? By having a friend "camp" the controller its possible to make distant pairings but the system seems to revert to green when the controller unloads.