I found the Trail at Tsuen King Wai, Tsuen Wan opposit & near Adventist Hospital. According to the information in Chinese web site, It takes about 5 hours to finish hiking. The trail had good view of Tsuen Wan and even Tsing Ma Bridge.
I did not try. According to the information, the trail via Ha Fa Shan (下花山), Shek Kuk Kung (石龍拱), Tin Fu Tsai (田夫仔) camp site, intersection of MacLehose Trail Section 10, then muddy road with full of trees towards Tai Tong (大棠) at Yuen Long which is a part of country park. At Tai Tong, there are public light bus & MTR Bus to Yuen Long Town Centre.
You can leave at Shum Tseng for roasted goose if you're tired for the whole journey.
Try it if you love hiking. I don't know if this trail really exist even before British rule.