Apparently, during the Victorian Era, these special carriages or "machines" were used to provide privacy to swimmers - mostly in the "proper" classes to keep men and women from seeing too much of each other during their summer outings to the beach.
I always knew about changing tents and buildings, but not about the machines.
During WWI, these machines were used to hold prisoners in Germany.
Here is a longer and more detailed description.
Just thought this was cool. How did I go this long without knowing about bathing machines?