Average of A B
This layout calculates the average of two numbers. These are added and then shifted right to divide by 2. Two functions are used:
 Add function, to add A and B into S.
 Shift Right function, to divide S by 2.
Operation
Enter the values of A and B and start the train from the station. When the train returns and halts, the average value can be read from register S.

Click grass to pause/run train  Click points to switch 0/1  Click start to reset train/points 
Lazy points switch between upper 0 or lower 1 branch lines. Trains arriving on a branch line switch the point to that line. 

Sprung points allow branch line trains to join the main line. All main line trains go straight ahead and never 'branch off'. 
Notes
 If the sum of A and B exceeds 15 then an overflow error occours.
 The value of B is 'lost'.
 No need to clear S before a calculation as it is always overwritten.
 The Shift Right function inserts leading 0's.