Just after "S" but well before "U" comes the letter "T". I have noticed that sometimes in signage, the letter can stand by itself to give information. On a certain steert there is a big letter "T", which may very well be a reference to the train. This video shows at least 2 other used of the letter "T".
https://www.youtube.com/watch?v=oLCoZZfhRx4