Big Question and Little Questions:
How many wagons is one horse able to pull along these rails at one time?
Why is this number greater than one horse can pull along an ordinary roadway?
What is valuable about the earth here in Purfleet that so much effort is made to dig deep for it into the ground?
How do the wagons stay on the rails?

For Information:
This is an early iron railway from the time before the widespread use of steam locomotion. Chalk was dug out of the ground and then burned in kilns, using coals brought by the nearby river, to produce lime. Lime was found to be useful manure, i.e. a substance added to the soil to make it more fertile.
The illustration comes from Arthur Young's books on Essex Agriculture,(1807), '25 horses were constantly employed: since these ways have been made four do the work, and 21 have been dismissed, which ate each half a bushel of oats per diem the year round…'
The rails seem to have been joined in pieces three feet long and were made in L-section to keep the wagon wheels from slipping off. They were in the style known as fish-bellied. The distance between the rails, or gauge, was three feet six inches.

Significance:
'Railways' existed before steam locomotion on a permanent way. Tracks or tramways have been found at other places like Tilbury Fort. The great change in transport of goods and people did not occur overnight. Many schemes were planned but not carried out. For example, H.R.Palmer, a founder member of the Institution of Civil Engineers, planned a horse and wagon 'road' using iron rails between London, Chelmsford, Colchester and Ipswich in 1825.

Artefacts and Documents for Evidence:
Tracks can still be seen leading into Tilbury Fort from the quayside.
See Essex Record Office document Q/Rum 1/33 for H.R.Palmer's 1825 scheme.

Further Reading:
Wood,R.G.E.,Railways in Essex until 1923,(1978, E.R.O. Seax Publication No.72), items 2 & 4;
'H.R.Palmer, A Forgotten Railway Pioneer' in The Railway Magazine, October 1953, pp.658-60.