Home | Design | Construction | Track Laying | Control & Electronics | Rolling Stock | Operations | Links | Contact
For a manually operated railway, block detection isn't important. However when automatic control is needed, it is necessary to know "where the trains are" so that a computer can decide if it is safe to move trains. This section captures information about the block detection system used.
The fundamental need is to be able to split the track into blocks, and monitor the presence of trains in each block. Each block must be longer than the largest train; movement from block to block sets the basis for automatic control. If the block ahead is free, a train can proceed; if it is not, a train must stop. it is logical for each block to have a signal, which tells the train driver if it is safe to proceed; on this railway, those signals are automatically set. That requires an ability to detect trains.
There are two quite different detector technologies:
-
Occupancy detectors determine, from the electrical current drawn, if a train is present in the track ahead. 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 detectors identify 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 has passed and 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 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.
Page Last Modified on: