This is a replacement for the photocells
that come with the Grade Crossing Pro and the Signal Animators. The
only possible need you would have for additional photocells for control
of one track at a grade crossing would be if your spacing was such that
a short train running too slowly might not hold the gate until the train
got there. In that case, a second photocell could alleviate that potential
(rare) problem.
It's not recommended, but these can
also be used for a poor-man's way of detecting a second track at a grade
crossing - by putting a second set of them in series with the others.
The only problem with this is that the CGP can get confused if two trains
pass at the same time - causing grade crossing event sequencing to be
out of sorts.
Sequencing will always be correct if
only one train passes at a time. It will only be affected when two trains
pass at the same time. When the first train triggers the GCP, it instigates
sequencing for that train going in the direction it's going. If another
grain comes along, going the other way on another track, the GCP won't
know that because it can't tell one track from another (That's what
multiple GCPs cascaded together are for). When the first train leaves
and the second train, going the other way, is still there, the GCP thinks
the first train is still there - it doesn't know that it's a different
train going in the other direction. You can see how this confusion would
cause sequencing problems.
But, if you don't mind this sequencing
problem when two trains pass at the same time, you can add detection
for a second track for the cost of only 4 photocells. But since you're
using Logic Rail components for realistic operation, we assume you will
want to do it correctly for the best results, with multiple GCPs.
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