How to use Railroad Presignals
What are Presignals
A Presignal is a special type of block signal. In a normal block signal, if no train is in the block ahead of the signal, it will show clear. In a presignal, the light will not show clear until the next signal past the presignal is showing clear. You could also say that the pre-signal shows the same characteristics of the next signal on the track.Look at Fig 1. below for a graphic demonstration of this. The top track has normal block signals, allowing the train to proceede until there is a train directly in the block ahead of it. In the bottom track, the presignal (blue hut with dwarf light) has stopped the convoy, because the block after the presignal is not showing clear
Fig 1. Comparison of Block signal and Presignals. Graphics from Pak 128, v 1-2-3.
Uses of Presignals.
The primary use of Presignals is to keep a train from blocking a junction. The junction could be a rail/rail crossing, or a rail road crossing, etc. There are some other useful things you can do with a presignal, such as strategically place them near inclines and declines to optimize convoys speed.Fig 2.Junction blocked by a train not able to leave the junction. Graphics from Pak 128, v 1-2-3.
In figure 2. above the junction is blocked because a train has entered and stopped in the junction. This unnecessarily holds up other traffic that could flow through the intersection. Figure 3. shows this junction fixed with presignals, and the better results it produces.
Fig 3.Junction now clear, since the presignal stopped the larger train from blocking the junction. Graphics from Pak 128, v 1-2-3.
Other good uses for Presignals.
Presignals can be very helpful in terminal stations located close to a dual track section. The two way signal before the terminal allows trains to both enter and exit from the same side of the terminal. But, if more than one train is visiting this terminal, the trains can deadlock at the two way signal. The two solutions for this problem are,- Set it up as a drive through (or RoRo) station, where the train enters and exits on different sides (not shown here).
- Place Presignals where the trains enter the juction block, as shown below in Fig 4.
When presignals are placed, no train will enter the junction block until it's exit from the junction is clear. This not only prevents deadlocks at the terminal, but can also keep a train on the mainline track from blocking the junction if it's track ahead is not clear.
Fig 4. Terminal station, with presignals guarding the junction block. Graphics from Pak 128, v 1-2-3.
As a further example, let us say that many trains are visiting this terminal. We would not want a train waiting to enter the terminal stopped on the mainline, preventing traffic from flowing on it. The solution to this problem is to build a siding track, where trains to the terminal can wait, place a waypoint in the trains Schedule to route trains onto the siding, and finally place a presignal at the point where the siding enters the junction.
Fig 5. Terminal station, with presignals guarding the junction block, and a waiting area. Graphics from Pak 128, v 1-2-3.
See also: use Signals Rail Plans
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