Symbolism, Imagery, Allegory
The modes of transportation featured in these stories help to characterize the 1890s and act as important symbols. Trains and bicycles are especially important in the universe of Sherlock Holmes. Trains were getting faster and were going more places in the 1890s, and we frequently see Holmes and Watson catching trains to solve crimes outside of London. In London, the duo usually takes cabs (of the horse-drawn variety) to get around town, but a ton of the characters in these stories opt for a new form of transportation: the bicycle. One of the stories featured here, the "Solitary Cyclist," even hinges on bicycles, and the "Priory School" features a bike chase.
There was actually a huge bicycle craze in the 1890s. New bikes were safer and were relatively cheap, so people of all social classes could own and use them. Bikes and trains help to represent how connected the modern world was becoming; people could move around faster.
Trains and bikes help to collapse not only geographic boundaries (people could travel all over the place pretty easily), but also class boundaries (both rich and poor people could travel). These ideas are also reflected in the crimes that Holmes and Watson solve. Their cases involve people of all social classes and nationalities and often occur in places outside of London.
Want to read more about transportation in Victorian London? Check out this cool site, a Dictionary of Victorian London, that has primary source articles on every topic under the sun.