In this discussion I will show how a wye can be signaled to maximize traffic flow but still de-conflict opposing traffic.

A Wye with prototypical ABS/APB signals:

You can treat the entire Wye as a single block but this drastically reduces traffic flow because only one train can be in the entire Wye at a given time and, without intermediate signals, you are not protected from opposing traffic and even then someone may have to back up in the case of a conflict (see: Why ABS or APB Signals Are Not Sufficient).

Signaling A Wye – One Block

To achieve higher traffic flow through the Wye, consider the following:

Signaling A Wye – Multiple Blocks

In my system the following configuration entries are made:

• A block consisting of the track in Apex A along with signals S1a, S2a, and S3a
• A block consisting of the track in Apex B along with signals S1b, S2b, and S3b
• A block consisting of the track in Apex C along with signals S1c, S2c, and S3c
• Track T1 is defined as a “Reservation Block” for the left head on signal S1a and the right head on signal S1c.
• Track T2 is defined as a “Reservation Block” for the right head on signal S1a and the left head on signal S1b.
• Track T3 is defined as a “Reservation Block” for the right head on signal S1b and the left head on signal S1c.

Note that T1, T2 and T3 are “sections of bi-directional track serving to connect two different blocks”, they are not blocks in the normal sense with their own independent signals.

The rules for “Reservation Blocks” are (in brief*):

• A Reservation Block may consist of one or more detected track sections.  It may be an actual block with signals or a ‘Virtual Block’ without signals.  It may span other blocks (i.e.: a multiple block ‘routing’). 
• A signal referencing a reservation block must reserve that block before giving a clear signal.
• Only one signal can reserve a given block at a given time.
• A block can not be reserved if it is occupied.

For a signal that references a Reservation Block (in brief*):  In order to present a Clear the signal must satisfy all the normal conditions for a Clear (train on the signal’s Approach Track, block unoccupied, turnout [if any] aligned for the route, signal selected as the next train to proceed through the block, etc.).  In addition, the signal must request (and be granted) a reservation on the Reservation Block defined for the signal before a clear signal will be displayed.

Since only one signal can hold a reservation at a given time, trains are routed through the Wye if there are no conflicting trains accessing the leg of the Wye they need.

For example, if there is a train “A” at S1a who wants to go to Apex C and a train “C” at S1c that wants to go to Apex A only one train will be cleared because both signals need to reserve T1 but only one can hold the reservation at a given time.

In the example above, if train C wanted to go to Apex B the two trains would be cleared at the same time because (except within the Apex C block) the trains are non-conflicting.  If train C has not cleared the Apex C block when train A gets there, signal S2b would hold train A until train C has cleared Apex C.

Thus, there can be three trains within the Wye at the same time, one on each leg and there is full protection for trains as long as they do not pass a Stop signal (all signals are absolute).  There are three of these Wyes on the test track in Florida and we have not had any issues, even at a large meet.

Note: Doing this did not require any additional relays or wiring, just configuration changes.

This could have been accomplished with APB blocks but it would have required as many as 16 more signal heads to provide the extra head end and intermediate signals but you still would not eliminate the possibility of a train having to back up see: Why ABS or APB Signals Are Not Sufficient).

* See the BC002 Users Manual for more information