Question about the roads around Washington DC during the winter of 1864-65

I recall that in the film "Lincoln" Seymore is shown talking to Lincoln about his worries that once the icy roads around Washington begin to thaw that the Congederates will attempt an attack. Can anyone provide me with some sources about the trails leading to Washington and their conditions during the winter? I'd like to learn more about this but I'm having difficulty finding information myself.
 
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