> Thank's Bob2 for the most appreciated UDE explanation.

To amplify on Bob2's explanation a little bit: when making a service application with AB/ABD/ABDX/ABDXL type valves, the operating piston in the emergency portion moves to its midpoint (essentially). In this position a port is uncovered to bleed quick action chamber pressure to atmosphere through an orifice which is sized so that QA chamber pressure drops at the same rate that the brake pipe is reducing. This action stops the piston in this position until the brake pipe stops reducing, at which time the chamber's residual pressure moves the piston just enough to close this port. Thus, each control valve carries an "up-to-date record" of current brake pipe pressure to use as a yardstick for measuring sudden, large drops in brake pipe pressure. In a sudden, large drop, the piston moves past it's midpoint to trigger quick action and send emergency reservoir pressure to the brake cylinder (where it joins air pressure from the auxiliary reservoir).

What was causing UDEs to occur during service applications is that some ports in the emergency slide valve and/or seat were too close together. With worn valves, the emergency piston was traveling a little too far in service applications, to a position in which quick action was being triggered. In ABDX/ABDXL valves, these ports were moved enough to solve the problem.

> I was trying to reason a reset(such as with the Vigilance device(Alerter) penalty brake application if tripped.

On US RRs, emergency applications and alertness control are two separate systems.

A UDE, actually any emergency application, "self-resets" (in a sense) after 60-70 seconds when the vent valve closes. This is a timing feature built into the vent valve's apparatus which ensures the train comes to a complete stop before the brakes can be released.

> So if you make a reduction, and the ude happens,

Game over.

> would you not place the ABV handle into Emg

Yes, immediately upon the UDE occurring.

>and then Release?

Not until the train has come to a complete stop and the trainmen have inspected the train for problems AND it's been determined that the train will not start moving on its own accord.

> Once you'd brought the speed down to a D/B holding speed?

You either keep trains speed to track speed or stop.

> I guess because the rules say you have to stop an walk,

This is because slack action in emergency applications all too frequently results in derailed cars.

> but you know you caused the ude by application,

That would be a DE (desired emergency).

> Back in NZR days this was the timing section, staying at Yellow over Red if you were going too hard and the next signal is the Departure Signal stop&stay with the Medium speed reduction displayed at Green on the bottom. If you were not getting it stopped here, the Departure stays Red over Red, the Points don't reverse and you end up on the catch track. Good luck there Mate.

I don't know of any arrangement like this on a US RR, which should not be taken to mean none ever existed. However, timed signals are/were quite common on rapid transit operations (NYC, Boston, Chicago, etc.).

> Thanks, there wasn't anything like that here, unless one described the old Searchlights rotating through the sequence on clearing.

Generally speaking, there were two types of seachlight operating mechanisms, ones which momentarily flashed red when changing from Y to G, and ones which didn't.