- Occupancy detectors determine if a train is present in the track ahead. They do this by monitoring the current fed into each track section. This requires that the track is fed with power in short sections, and that each feed has a current detector. These, nowadays, are sensitive enough to detect a loco that is present but not moving.
- Position detectorsidentify the presence of a train at a particular location. This might be an optical detector (either using reflected light, or an interrupted light beam), a microswitch, or a reed relay operated by magnets. These can reliably indicate that a train is here right now. However they provide no reliable indication that a train is between one location and the next. Some manufacturers have provided ways to join them together, so that a train passing the next detectors cancels the indication from the previous one: but it is essential to make sure that such a scheme can't be fooled by human hands near the rail etc.
On my railway there is a need for both kinds of detector. Occupancy detectors are great for identifying that there is a train ahead, but not exactly knowing where. This is good to detect if a signal is red or not. Position detectors would be good at identifying the exact time a train reaches a location. This would be good for stopping at a specific position - e.g. in a station or at a signal.
In practice, I've selected occupancy detectors for both roles. A "main" block detector covers the majority of the path to the next signal; a short section covers the piece of track immediately in front of the signal to tell a train to stop if it is not safe to proceed. Some research into stopping distances identified that the train would typically stop within 40-100mm of a new occupancy detector region, depending on speed. In practice the speed would be low by then, with a smaller margin of uncertainty.
I've mostly used Digitrax BDL168 block detectors. A single board provides 16 separate detect channels, split into groups of 4 (each group may be in a different power zone, but all 4 within a group are connected to a single power zone feed). Some of the predecessors of this product had issues with some kinds of loco decoder; but the newer ones are pretty reliable and I've seen them in use with no problems on other computer controlled railways. In the fiddle yard, a late design change resulted in a need for a few extra detectors. The Digitrax BD-4 has been used to provide 4 detection channels, and connected to LocoNet by SIGM20 units.