Historically, only members of the royal family and the King's Troop, Royal Horse Artillery, can pass through the arch in ceremonial procession.
The brougham (pronounced 'brohm' or 'broom') was invented for jurist Henry Brougham, or became very fashionable by his use of it. It was a four-wheeled horse-drawn carriage that could seat two within its enclosed body and sometimes had a pair of fold-away seats in the front corners.
It had a seat in front for the driver and a passenger. It also had a glazed front window, so that the occupants could see forward. It was a light vehicle, and the forewheels were capable of turning sharply.
A quote from William Shakespeare's Romeo and Juliet.
If I profane with my unworthiest hand
This holy shrine, the gentle fine is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.
Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims' hands do touch,
And palm to palm is holy palmers' kiss.
Act I, Scene 5
These are the first words exchanged between the two lovers.