Railways in Britain are mainly measured in MILES and CHAINS.

80 CHAINS = 1 MILE = 1760 yards = 1.6093 kilometres
1 CHAIN = 22 yards = 20.117 metres

The mileposts on the RDHR are calculated from a nominal point
almost 7 miles west of Custom House, and range from 6m72ch at
Custom House itself to 8m57ch at the North Woolwich terminus.

This means that the RDHR is technically 1m65ch (2.917km) long.

Since the measuring point at intermediate stations like Custom House
is usually midway along the platforms, the operational length of the RDHR,
including the full length of the platforms at Custom House (plus a short
shunting neck beyond) is slightly more than this a little over 3km.

Photographs in See the line show the approximate milepost
value at each location. In some cases this has been estimated.

red boxes mark the rough position of the photographer.)