The diesel locomotives (and railcar when available) are used first, with the steam locomotives being used on trains as required. On arrival at Llanberis, diesel-hauled trains run directly from the arrival platform to the departure platform, load and depart within a few minutes, making one trip every two-and-a-half hours. Steam-hauled trains take at least half an hour to transfer from the arrival to the departure platform, thus making no more than one trip every three hours.
The use of diesel locomotives therefore allows more trains to be run with the same number of carriages. By using diesels, the reduction in costs for both operating trains over the line and having them standing between infrequent runs has allowed the operating season to be extended considerably.