The common miniature railway gauges in the UK are: 1.25" (approx) AKA 'O' gauge, 1.75", 2.5", 3.5", 5" and 7.25"
Gauge is the distance between the inner faces of the rails.
It looks like yours are 2.5" gauge.
The smaller gauges were very popular in years gone by as the materials cost & equipment needed to make them is small.
Boiler certs: Most model engineering clubs have a few nominated members with the skills to issue boiler certs.
Visual inspection (and during construction if new build), hydraulic test, steam test.
This can be difficult if the design is not one of the known published designs.
HTH.
Good luck with the sale.
Gauge is the distance between the inner faces of the rails.
It looks like yours are 2.5" gauge.
The smaller gauges were very popular in years gone by as the materials cost & equipment needed to make them is small.
Boiler certs: Most model engineering clubs have a few nominated members with the skills to issue boiler certs.
Visual inspection (and during construction if new build), hydraulic test, steam test.
This can be difficult if the design is not one of the known published designs.
HTH.
Good luck with the sale.