大欖隧道
On 10 August 1992, a special service, 51P, was started, from Shek Kong swimming pool to Tsuen Wan. In 1996, KMB assigned two air-conditioned Dennis Dart buses (AA) fitted with manual decelerators to Route 51. In the same year, the District Councils of Tsuen Wan and Yuen Long planned to divert Route 51 to Tai Lam Tunnel once it had been completed with terminus relocated to Sheung Tsuen via Kam Sheung Road, but to avoid loss of bus service on Route Twisk, KMB ended up creating a new route numbered 251M. Patronage on 51 dropped so much that 51P was discontinued.