"On the platform there's a machine that lets you print your name onto a strip of aluminium, threepence for up to ten letters."
Were these name printing machines still around by the time British currency was decimalised in the early 1970s? - perhaps this example pictured at York Railway Museum may have been adapted for the museum's commercial use, hence the 10p price tag. It's otherwise exactly like the one the boy uses in the story.
Most of these machines were made for the railways by the British Automatic Company, who also made platform ticket dispensers. In those days, if you were visiting the station but did not intend to travel (train spotting for example, or even just seeing someone off) you had to purchase a platform ticket which normally cost 1d or 2d.