If you want to signal a siding for occupancy detection it requires six signals:
- an exit signal in the siding at each end
- A home signal on the main track at each end for facing-point movement over the turnout.
- An absolute signal on the main track at each end for trailing-point movement.
Most modelers merely place signals in locations where their change of aspect can be seen without actually adding train detection. These signals automatically clear themselves on a timer after the train passes.
If you want only switch position protection, then you could do it with two opposing signals on the main track, one at each facing-point location. They can additionally be set up to be train activated and to self-clear after a train passes, but, whenever the switch is lined for the siding, the signals will display Stop and Proceed.
I have train detection, but I also have a $3500 investment in signals, relays, and cables from Custom Signals, beautiful stuff, but it requires a lot of equipment and many insulated blocks in the track. There's so much involved in this that there are very few model rail automatic block signal systems. I'm not trying to rain on your parade, but am just encouraging you to think about whether you are passionate about functional block signals. Most modelers, especially in 3-rail, do not really want train detection, except possibly in isolated sections where visibility is impaired.
If you just need something as a visual reminder of switch position at some location where it's hard to see the switch points, then a Z-Stuff semaphore is your best bet. You can always see the position of that semaphore blade.
I hope you find the right solution for your needs, desires (and wallet).