From the Ring website:
"The new LM-3S and LM-3 Locomotive modules have twice the program memory and the LM-3S has twice the storage memory. Extra program memory will allow us to add more features and improved sound with program updates. Extra storage memory will allow more sound files, pictures, and light effect files to be loaded! Also instead of a 2nd input like on the LM-2, the new LM-3 has a 'Negative' terminal.
Is better for battery powered connection and will be more flexible to add a keep alive product."