When passing through Kowloon Tong station, if you walk up the northbound platform you will find the 4 1/2 milestone marker.
The Kowloon-Canton Railway has been through several realignments and extensions since it opened in 1910, but one of the most significant was the double-tracking and electrification work, since it would allow trains to run in both directions. Unfortunately, the Beacon Hill tunnel connecting the New Territories and Kowloon was just too small, so a new tunnel was dug in 1981. You can see how the original tunnel is to the right of the current permanent way.
After Kowloon Tong Station was built in 1982, the way was realigned to continue through the new tunnel. The original tracks curved to the right, through the now wooded area, before entering the Beacon Hill tunnel. The original tunnel is now used as a conduit for a gas pipeline, but if you have the keys, it's possible to walk from Kowloon to Shatin through the mountain.
Unfunny story: during the Battle of Hong Kong, a group of retreating soldiers did just that from the New Territories to Kowloon ... only to find that sappers had blown up the exit just hours before to slow the Japanese advance. As Philip Cracknell relates,
"The Royal Engineers were ordered to carry out a demotion at the Beacon Hill railway tunnel to prevent Japanese troops from using it to gain access to Kowloon ... during the evacuation, one section of two 3.7 inch howitzers ... withdrew under orders through the railway tunnel. As they reached the Kowloon end of the tunnel, they found the sappers had completed their demolition task and the tunnel was completely blocked. They had to retrace their steps back through the tunnel, and then take the hill tracks through Railway Pass, east of Beacon Hill, to Kowloon."