doll

A doll is a small signal post or arm mounted subordinately on a main signal structure, either bolted to the mast of a primary signal or attached to a signal gantry. It displays supplementary indications that modify or qualify the main signal's instruction, rather than replacing it. The doll occupies a fixed position relative to its host structure and is typically much shorter and lighter than the primary signal it accompanies.

The most common use of a doll is to carry a subsidiary aspect that applies only under specific conditions or to specific routes through a junction. For example, a doll mounted below a main signal might display a "proceed with caution" aspect that applies only when a particular points setting is in place, or only for trains taking a specific diverging route. This allows the signal engineer to communicate conditional clearances without requiring a separate standalone signal post.

Dolls appear in various configurations depending on signalling system and era. In mechanical signalling, a doll might carry a small semaphore arm or a miniature disc. In colour light systems, it typically displays one or more aspects on a small multi-aspect unit. The mechanical or electrical linkage connecting the doll to the points mechanism, or to the main signal's interlocking frame, ensures that the correct aspect appears only when the corresponding route is properly set and locked.

Where dolls fit in the signal landscape

A doll saves cost and complexity by avoiding the need for an additional signal post at a location where space is constrained or where a supplementary instruction needs to be grouped physically with the main signal. On a busy junction with multiple routes, dolls can reduce the visual clutter of the lineside while still conveying all necessary aspect information to drivers. The term "doll" itself is primarily British railway vocabulary; other traditions may use terms like "sub-signal" or simply describe the component by its function.

Maintenance of a doll requires access to the main signal structure itself, since the doll cannot be serviced independently. The mechanical or electrical components of a doll age and deteriorate in the same way as any other signal component, and in older installations mechanical linkages may require regular lubrication and adjustment. Modern resignalling programmes often replace mechanical dolls with integrated multi-aspect colour light heads that eliminate the subordinate mounting altogether, consolidating all aspects into a single unit.