Theory:

The pattern of rhymes at the ending of each line in a poem is called a rhyme scheme. Letters (A,B,C...) are usually used to express which lines rhyme. Verses that are designated with the same letter are said to rhyme with each other. It is also known as an arrangement of rhymes in a stanza or a poem.
Example:
1. For easier understanding of the concept, let us take a famous nursery rhyme, written by Jane Taylor as an example.
 
Twinkle twinkle little star,
How I wonder what you are.
Up above the world so high
Like a diamond in the sky.
 
We see that the first two lines rhyme with each other (star-are); the second two lines rhyme with each other (high-sky). Let us name each line as \(A\), \(B\) depending on the words that rhyme with each other.
 
Twinkle twinkle little star,
\(A\)
How I wonder what you are. \(A\)
Up above the world so high,B
Like a diamond in the sky. B
It can thus be seen that this poem follows \(AABB\) pattern.
The rhyme scheme of the poem "From a Railway Carriage":
Faster than fairies, faster than witches,
A
Bridges and houses, hedges and ditches;A
And charging along like troops in a battle,B
All through the meadows the horses and cattle:B
It can thus be seen that this poem also follows \(AABB\) pattern.