If you have Tane sp1 or above, the built-in Crowcombe-Heathfield Signal Box has the IT functionality. The kuid is <kuid:401543:3220> and is for Trainz version 4.3 and above.
An alternative is to use the TRC 3 Invisible Interlocking Tower (downloadable from DLS). It is only visible in Surveyor so could be placed next to a signal box, or any other scenery object, of your choice.
Note that if you have not patched up to SP1, you won't be able to use the Interlocking Tower functionality.
Regards,
John
