Even where unit lamps are employed stacks of up to 6 units are not uncommon to create two 3 lamp heads such as those seen here at Amtrak's COVE interlocking.
The thing is...what if you didn't feel like spending that much money on all those pesky signal heads? A virtual signal head is where what would normally act as a single signal head instead serves as 2 or more using the relative placement of lights without the associated spacial placement. This is generally seen as being a rather severe form of cost cutting and is rare even in North America where signaling hardware is already fairly austere. One of the more prominent main line examples is on the former Milwaukee Road main line between Milwaukee and St. Paul, now operated by CP and uses 4-lamp masts to display the full range of weak route signal indications like Y/Y, R/G and R/Y.
Or a 5-lamp mast at interlockings where both Y/Y and L aspects are required.
However the cost effective signaling award might just belong to Conrail for its use of virtual headed 2, 3 and 4 lamp dwarf signals. Now this might be more widespread and I'm probably being a bit unfair picking on Conrail, but on the other hand the NORAC signal aspect system was tweaked specifically for just this sort of low cost signaling so I suspect that Conrail was one of the more aggressive users of the practice. How does NORAC cater to the use of Virtual Heads in dwarf signals you ask? G/*R* for Medium Clear and Y/*R* for Medium Approach avoids having aspects like R/G or R/*Y* create conflicts with a virtual head system. All NORAC dwarf indications can be displayed on a signal with Red on the bottom.
|Y/*R* Medium Clear at CP-TRENT Yes, its flashing.|
Conrail went crazy with the concept. A 2-lamp dwarf displaying only Stop, Restricting or Slow Approach could now display Medium Approach. Adding a green lamp gave them Medium Clear, Approach Medium and Approach Limited. Limited Clear was given the coveted *G* dwarf slot so that doesn't really count as a gain, but the C&O was forced to use *G* for Medium Clear on their non-virtual US&S N-3 signals so the extra flexibility is still there. The only two dwarf indications not covered were Y/Y Approach Slow and G/G Clear, with both requiring a 4th lamp (Clear AND Approach Slow would require 5 lamps). Again, while one might assume the signal engineers would want to put a gap between the greens such as G-Y-G-R that would be an incorrect assumption, especially where a 3-lamp is getting an upgrade to 4-lamp, such as a Rule 251 route going 261.
|Because rearranging signal heads costs money.|
On the NJT RiverLINE NJT was responsible for the initial signaling design which resulted things like this 7-lamp monster at the Trenton Station.
|I think this has to be a record...no wait, there's DART.|
Which even maintains a virtual R/R as seen below.
There are other instances on the RiverLINE of huge 5 and 6 lamp signals being converted into more compact varieties such as a Y-R-Y-R unit designed to give Approach Slow.
Let's take a quick look at some other popular speed signaled railroad. In this rather unique Canadian example we see a US&S N-2 dwarf attached to an H-2 searchlight to create a 3-lamp dwarf signal capable of displaying Restricting and all the other slow speed indications.
Because of its Lunar White fetish, the use of 4-lamp dwarves on CSX is common, but unfortunately its use of R/Y and R/G for the "Medium" indications means it is harder to employ virtual heads. Of course it does make it easier to tell if turnouts are signaled for slow or medium speed such as this example on the re-signaled former Conrail Selkirk branch at CP-SK. Had CSX stuck with NORAC an upgrade would have not needed any hardware changes.
|Typical slow speed CSX 4-banger dwarf. Don't expect more than one of these to light up at a time.|
As for the former Conrail parts of NS, I am sure that the virtual head system is still in use, but I also know that NS seems to be much more committed to using signal masts, even in instances where a simple 3 indication dwarf would do such as here at CP-WORKS.
|You really needed those 2 full size red marker lights?|