Dating to the 1950's, this unusual instrument by Negretti & Zambra is designed to gap train rails allowing for expansion according to ambient temperature during installation.

Comprising of a fahrenheit thermometer encased in a copper block, it's a highly conductive device which measures expansion from nil to 3/8 inches. Although 60 foot rail lengths were introduced in 1929 to replace the 45 foot standard, this particular example is engraved "BRITISH RAILWAYS" on the underside, indicating it was made sometime between 1948 and 1965, when British Railways started trading as British Rail.

Housed in an oak box measuring 6"x2"x1" the weight is just under 300 grams and in very good condition with the thermometer in good working order. Interestingly there is also a paper inlay on the lid underside with a temperature guide for both the modern flat bottom rails, and the older bullhead rails which were discontinued in the mid 1950's.