My wife and I left from Georgetown to Harpers Ferry then up to Antietam in early July on a loaded tandem with 35c's. North of Seneca Falls the trail turned noticeably more difficult even though it hadn't rained for several days before we arrived (we lucked out!) though the week prior there was lots and lots of rain. Still, we bailed at White's Ferry and took the W&OD instead because of the 15-20 miles of total muck above Seneca Falls to White's Ferry simply wore me out. It made for some interesting riding. We fishtailed a whole bunch and nearly lost it once or twice. We had to walk through several patches of ooze.

It seems that from Georgetown to Seneca the trail drains better because it's mixed with crushed gravel and is more open to sunshine. Above Seneca the trail became clay-like and unable to drain. Plus, the tree coverage is much more pronounced.

The following day from Harpers Ferry to Antietam was great.

Overall, I'd think you'd have a real struggle with a loaded tandem on the C&O with rainfall a day or two prior or during your trip. I had multiple Plan Bs available just in case and ended up using one (the W&OD trail) even though we picked the very best time for the ride, weather-wise. You'd do well to plot alternative courses just in case.

One final note: the C&O Canal itself was truly awesome. We loved it, these issues notwithstanding. Have fun on your trip.

Andy