What: The men are digging away Morrison Hill, so that the rock and soil can be dumped in the sea to form the large 1920s Wanchai Reclamation.
Given the scale of the project, it's remarkable to see the excavation is done by hand. Here's a man working at the top of the hill, digging away with his pick:
The rock and soil runs down a channel, and ends up in this temporary silo at the bottom of the slope. They've cleverly left a wall of rock at the bottom, with V-shaped holding areas. The one in this photo is blocked with a couple of boards:
Under the boards is a long plank, stored flat along the rock wall. And just below that you can see a groove cut into the rock, just above the lip of the truck.
Over to the right, we can see what these are for. They take the plank down and slot it into the groove. Then the retaining boards are removed, and the soil pours out. The plank makes sure all the soil ends up in the truck, instead of falling down the gap between truck and rock.
When the trucks were full, they'd run along tracks laid next to the Bowrington Canal, then out onto the reclamation area where they'd be emptied out.
Who: The side of each truck is painted with its number, and the contractor's name: "Sang Lee".
The company is introduced in the document "Praya East Reclamation Scheme. Final Report." :
In 1921, a Contract [...] was awarded to Messrs. Sang Lee & Co., whose tender was the lowest at $2,397,323.00 [...]
Sang Lee & Co. are mentioned in most years' Annual Reports for the Public Works Department between 1900 and1940. They must have been one of the largest contractors of the time to take on a project this size.
Where: I'll go with Morrison Hill as the site, based on the information above. However Sang Lee & Co. worked on many projects, and other construction projects also used railway trucks to move building material. If you think the photo shows another location, please let us know in the comments below.
Morrison Hill covered a large area, so can we narrow it down any further?
If you look at the close-up photos if the trucks, you'll see there are several large wooden pegs sticking out from the rock face. Usefully for us, they cast a shadow. We can see the sun was high in the sky, and that the pegs point towards the sun - so somewhere between southeast / south / southwest in direction.
I guess the photographer was on the old section of Queen's Road (previously called 'The Gap') roughly opposite where Wanchai Park is today. He was looking north towards Morrison Hill, where they're excavating northwards towards the sea. The railway trucks would run east to the right of the photo, down the hill to Bowrington Canal, and out onto the reclamation.
One clue that may help confirm this - there's a building visible on the top of the hill. It isn't very clear, but can anyone recognise it?
When: Probably the early 1920s. The report  again:
When the Contractor had been operating for about 1 1/2 years large crops of boulders appeared in the faces of the cuttings, the removal of these boulders not only added considerably to the expense of obtaining filling but slowed down the rate of progress to such an extent that to maintain progress consistent with completing the work within Contract time, it was necessary for the Contractor to operate late into the night. Conditions gradually became worse and the Contractor reported in March, 1924, that he was unable to carry through the work at his contract rate of $0.23.
(The government eventually ended up paying a bonus of 76% to enable the Contractor to complete the work.)
There isn't any sign of the 'large crops of boulders' in this photo. I'll guess it was taken before they appeared, around 1922.
Lots of guesses! If you can add any more facts to the story, please leave a comment below.