The railheads below go with 12mm round bar and 30mm by 10mm rails.
The railheads below go with 12mm square bar and 30mm by 10mm rails.
The railheads below go with 16mm round bar and 40mm by 10mm rails.
The railheads below go with 16mm square bar and 40mm by 10mm rails.
The railheads below are replica Victorian heads from around London and are made with 20mm round bar and utilize 50 by 10mm flats rails.
Here is an example of the LS3 Victorian replicas drilled into the plinth.