From Dave Althoff Jr's Ride Technology pages
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The PLC is a special-purpose computer which simply takes real-world signals, makes decisions based on those signals, and produces output signals based on those decisions. In its most basic form, a PLC program operates in exactly the same way as relay logic. But the PLC is more flexible than a bank of relays. For instance, a PLC can easily be equipped with a switch to count brake fins as a train goes by so that it "knows" all the cars are accounted for. It can count lap bar release pedals so that it can detect a bar left open. It can be connected to an anemometer to measure the wind speed and set brake pressures accordingly. The PLC can even detect train speed at various points on the course and monitor the ride's performance. These days, most roller coasters use PLCs to control the block system, and the rule remains exactly the same as always: No more than one train may occupy any block on the ride at any time. The difference is that the PLC has the flexibility...because it is programmable and because it can do advanced processing on its inputs...to also automate and optimize other functions on the ride, and to monitor the ride's critical systems. The PLC has made ride controls idiot-resistant, and simplified the process of diagnosing system faults.
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The Allen Bradley unit is a human interface for the PLC - handy for keeping logs of incidents/incursions/downtimes/resets... that sorta thing.