Here is the amount of rail trackage (in kilometers) by country in 1900:
Austria-Hungary - 36,330
France - 38,109
Germany - 51,678
Great Britain - 30, 079
Russia - 53,234
There are a lot of ways to assess what this means. In terms of trackage compared to the size of the country, Russia was probably the least developed. In terms of trackage per population, Russia was probably more developed. In terms of rate of growth heading into the 20th century, Russia was the clear leader.
In trying to figure out which country's rail system was the best developed, I think the answer might differ depending on whether it's assessed for a military or a commercial application. Britain's system was the most highly evolved for commercial use. Being the oldest (in 1840, 60% of all European rail capacity was in England), the burgeoning industry of the latter 19th century could choose locations with good rail access, so it was most tightly linked to manufacturing efficiency. However, since Britain was an island, its rail system was utterly useless for delivering troops and supplies to a continental war theater.
Overall, I would say that Germany had the advantage with its rail system -- at least militarily speaking. The French rail system, which was built under the oversight of Paris bureaucrats, was highly centralized, with all major lines converging on Paris. Consequently, there were few direct links between regional centers. The German system, which was privately developed, was much more decentralized, with numerous regions developing their own systems. Consequently, cities like Hamburg, Berlin, Dresden, Munich, and Frankfurt had direct links with each other, and goods could move around the country very efficiently.
The Russian system lay somewhere in between. The early lines focused on St. Petersburg and Moscow, but prior to World War I lines were beginning to be laid with more of an eye to economic development and access to raw materials. But the process was still in its infancy. (One of the reasons for Russia's sorry showing in the 1904-05 war with Japan was the lack of a completed rail line to the east by which troops and supplies could be delivered to the war theater.) Also, the western powers began to suspect Russia's motives as more and more lines were laid heading to Russia's western borders -- a trend from which the western powers could only infer a military purpose.
A major limitation on the Russian system in war was that it used a different guage from the rest of Europe. So if Russia wanted to move men and material by rail into a western war theater, she either had to put them onto different trains (presumably captured from the enemy) or to change the axles and wheels on her own trains at the border -- both inefficient and, in wartime, somewhat dangerous.
I think the Russian rail system, in terms of how it was being planned and built by 1900, was headed toward being a reasonably efficient system. However, the acid test came in World War I when it ultimately failed to keep the troops and the cities supplied with weapons, food, and fuel . . . which, in turn, was a key trigger of the February revolution. That failure, however, was not brought about by inherent design flaws in the rail system, but by graft and incompetence among the civil and military authorities who ran the system.
