You may think its strange to call the nearby bridge an arch when its not,
The old arch bridge was only big enough for a single decker bus to pass under in the middle. As traffic got heavier they decided to change to the one you see today.
To build the new one they built the 2 sides then stopped traffic one night and blew up the middle section then dropped in the new top section..
Older People still think of 'under the arch' whilst passing under it today
There is no need to go onto the bridge or railway line, watch out for big spiders guarding the cache