You may have seen this on the train while coming into St Albans. It's the St Albans South Signal Box, a free-entry attraction suitable for all ages.
If you haven't been this is a must visit, it's fantastic! It's run by some very lovely and very knowledgeable people who are more than happy to show you around. You can operate the signals and ring bells, and browse the historical artefacts. You can watch miniature trains chug along a miniature track in the garden model railway and the big fast trains whizz by on the big track. You can even get a cup of tea.
It's free although a small donation is appreciated. Find it at the end of Ridgemont Road by St Albans City Station.
It's only open from 2-5pm on various Sundays throughout the year. Check the website for latest opening times.