Troika Songs

At the end of the eighteenth century, beginning of the nineteenth, the primary means of transport in Russia were the troikas, translated "threesome" refering to the use of three horses. The troikas were mainly used for postal services and transporting passengers, mostly for long distances, which was certainly the case for the traject Moscow - St-Petersburg. The Russian troika was known for its high speed (up to 50 kilometers an hour) and was often compared to a flying bird. The high speed was possible due to the special training and arrangement of the horses at the carriage. The horse in the middle, called the shaft horse, trotted and acted like a locomotive, while the two horses on its sides, the tracers, maintained a brisk gallop. 

This method of arranging the horses became popular not only for its speed, but also for its reliability, good cargo capacity, maneuverability and safety. The three-horse carriage was much more stable than the two-horse carriage that was used throughout Europe.
Driving a troika was a hard job and those who wanted to become a troika driver, were trained from a young age on and needed to be physically fit, as their arms sometimes had to endure up to 50 kilograms of weight, while trying to maneuver the horses. Because of the special arrangement, they also could control the load on each horse and at a certain point they could lash one side horse, so that it would run faster and let the other rest. 
The coachmen were also exempt from capitation taks and their profession was hereditary and prestigious among the peasants.

Yet it was not only the troika driver with the horses that appealed to the imagination of the passersby, the whole picture was not only impressive, but also nice to listen to. As the troika became widespread, the creation of special signals in order to regulate the troika traffic became necessary. As they moved at very high speeds, especially the postal troikas, and there were no traffic rules, sound signals were develloped.

At first the coachmen sent out a deafening whistle to let the next postal station know that they were approaching, so that fresh trotters could be prepared (this were the best troika horses, very fast and elegant). This warning was later banned and replaced by shaft bow bells, which became a part of the Russian troika. The valdai bells were particullary famous, producing a clear, singing sound, so that some compared the Russian troika to a musical instrument.
Usually the troika was accompanied by about 100 bells: on the bridle, the saddle and inside the many harness brushes. The singing of the bells, some in minor, others in major, combined with the sound of the running hooves, created a unique melody, pleasing to the ear. Moreover, troikas were also a joy for the eyes, with their richly decorated harnesses and bows adorned with painting and fretwork.

Sources:
http://russiapedia.rt.com/of-russian-origin/troika/
http://www.vadimprokhorov.com/russian_folk_songs__musical_genres_and_history_25733.htm
Oxford history of Western Music: Richard Taruskin, music in the 19th century, peasants and history (Russia)