garethwilkins:
Thanks for that. Looks like if got a bit of a clue now.
Was after midnight when I wrote post#2.
Here are the calculations, assuming the Arduino supply is 5volt and the train is +13volt and -13volt.
13 - 5 = 8volt falls across the top resistor (input) of the divider, 5volt across the resistor to ground (the Arduino).
So the divider ratio is 5:8.
If you pick 10k for the resistor to ground, then the top/input resistor calculates to 8/5 * 10k = 16k (E24 value).
The resistor from pin to VCC (5volt) must be the same value as the first two in parallel.
(10*16) / (10+16) = 6k15.
6k2 is a standard E24 value, and close enough.
Leo..