If Japan is unable to defeat Russia in a quick campaign then it is hard to see how adding Russia to its list of enemies is rational.
Japan really has some experience in this area. In the Russo-Japanese war they took Manchuria from the Russian Army. In 1919 the IJA was the principle Allied force (with some US Marines!) in the Intervention in the Far East (other forces - mainly British - landed at Murmansk and still others in the South). IJA got to occupy the rail corredor in this area and knew it well. Japan also fought the Soviets twice in the late 1930s (once on the Manchu-Mongol border at Nomanhan/Khalkan Gol, the other time on the Korean border with Russia), and had a clue what it would be like.
IJA had no hope of "defeating" Russia in a strategic sense (occupy the whole country) in a long term campaign. Not one of its operational plans - during much of WWII these were revised by Gen Yamashita who served as Kwangtung Army commander - contemplates operations West of Lake Baikal. Instead, Japan hoped to take Amur Province and the area to its immediate west, setting up its Western HQ at Chita. This is the bottleneck - the only area where there are no lateral lines of communications for an extended distance in mountainous terrain. They believed - and I think they are right - that taking this area would allow them to "cork the bottle" - that the Russians could not bring enough power to bear to be unmanagable on a broad front. They also believed, and again I think they are right, that the forces in the East were vulnerable to cutting their lines of supply. The main line was by sea to Vladivostok - obviously not viable if Japan was the enemy. The other line was the RR - and it could be cut at many points. The Russians can put as many troops as they like in Amur Province - but can they feed them?
