How early should I purchase an Amtrak Bedroom?

[dgvrengineer]

I am traveling from Chicago to Culpeper, VA in a roomette in May. The Capital Ltd was $171 including taking the Roanoke train from DC to Culpeper. The Cardinal was $353.  I watched using AmSnag for weeks, then one day the Cardinal dropped to $251 so I grabbed it (I prefer the Cardinal for the eastbound views). Two days later it went back up and so did the Capital. Not sure why it dropped for a couple of days, but it pays to check frequently.