Theory:

Here is a child who clambers and scrambles,
All by himself and gathering brambles;
Here is a tramp who stands and gazes;
And there is the green for stringing the daisies!
Here is a cart run away in the road,
Lumping along with man and load;
And here is a mill and there is a river:
Each a glimpse and gone forever!
Explanation:
 
Next, the poet saw a child who was trying to climb up trees with great difficulty, using both hands and feet - in an awkward manner.
 
clambering.jpg
"Here is a child who clambers and scrambles!"
 
The child was trying to pick all the blackberry fruits from the tree. The brambles have thorns and can be prickly - the child was all alone, scrambling carefully.
 
brambles.jpg
Brambles - a prickly blackberry shrub with thorns!
 
He gathered blackberries from the tree.
 
gathering brambles.jpg
Gathering blackberries!
 
The poet saw a homeless person who was standing and looking intently at the things happening around him. The poet means we do not have time to stand and look at the things happening around us in the world. But the tramp, a person who travels in search of work and food, has no time restrictions - he spent his time observing the small pleasures around him.
 
tramp.jpg
A tramp was gazing intently at the things happening around him!
 
The poet saw a piece of grassy land, as his train moved along. There were people making garlands of daisy flowers.
 
stringing daisies.jpg
Stringing daisies!
  
Then he saw a cart with full load, moving slowly and heavily on the road.
 
cart.jpg
A cart lumping along with load!
 
Finally, he also saw a mill and a river along the tracks of the train.
 
mill+river.jpg
"And here is a mill and there is a river"!
 
The poet saw all these visuals just for a moment. He could only take a short glance at it, as the train was speeding away. He could see it once, and it was gone forever - meaning he cannot return to see it again, as the train had passed and moved along on its way.
 
Meanings of difficult words:
  
No.
Words
Meanings
1
clambersclimb or move in a difficult manner, typically using both hands and feet
2
scramblesclimb up quickly, awkwardly, to clamber
3
bramblesa prickly shrub, especially a blackberry
4
trampa person who travels from place to place on foot in search of work or a beggar
5
gazeslook steadily and intently
6
green(here), a piece of public grassy land, especially in the centre of a village
7
stringinghang so that it stretches in a long line, make a crown/garland (of flowers, here)
8
lumpingcarry (a heavy load) somewhere with difficulty
9
glimpsea momentary view, a glance
Reference:
State Council of Educational Research and Training (2019). Term-2 English Standard-6. From a Railway Carriage - R.L. Stevenson (pp. 128-131). Published by the Tamil Nadu Textbook and Educational Services Corporation.