Most armies have contingency plans, and the Germans are famous for the years of planning that went into the Schlieffen Plan. But what if Germany went to war against just Russia with no France involved? We're any plans drawn up for such a war? What tactics would the Germans have employed for such a war? If they didn't have plans why not? This question is for between 1840 and 1930
The period you are referring to is one of massive technological advances as well as a rapid evolution of army doctrines, so it's hard to answer this in a broad stroke.
I can tell you that in the case of the early 20th century they did consider attacking Russia first (or only if that was ever possible). But what the high command realised is that a Schlieffen Plan style massive invasion would not be possible because the railway infrastructure in Eastern Prussia was not well enough developed. This was certainly also true for any fast advances into Russian territory.
Interestingly enough, the Germans were also really scared of Russia building a dense railway network because they knew what that means militarily. And Max Hastings argued that this was the reason they wanted war with Russia sooner than later.
Sources: Official German History of World War I and Max Hastings' Catastrophe
Could you be more specific to a single period or event, because if you are referring to the schlieffen plan an attack on Russia would be impossible as France and Russia had a defensive alliance leading up to the conflict of WWI. Therefore it would be impossible to just attack Russia as France declared war as a response to Germany declaring war on Russia.