Allow me to quote from John Corin's "Levant - A Champion Cornish Mine".
The 278 fathom level was the main tramming level of the mine, and it was thought that a small industrial locomotive by Bickle of Plymouth could perform the job better than hand-tramming .... The locomotive was a failure, however, having insufficient adhesion to haul trams of ore on the wet rails in the 278. So shortly after its trial in 1872 it was replaced by pit ponies, and the 278 was renamed the Pony Level