I need information from one of the German train travel experts

I have a eurail pass for Germany and Austria.

I want to use the night trains on two occasions, from Hagen to Berlin and from Frankfurt to Vienna.

On the Deutsche Bahn website they give you a option to only buy accommodation on night trains if you have already a ticket, does this refer to a eurail pass also?

Yes. The rail pass covers the basic train fare. On most overnight trains, passengers pay the normal train fare plus a supplement for sleeping accommodation. Someone might buy a normal train ticket for a journey, then decide they want to use an overnight train, and pay this supplement for sleeping accommodation. If you have a rail pass, you have to pay only the supplement.
Sleeping accommodation is usually in four- or six-berth couchette compartments, or one-, two- or three-berth sleeping compartments. Pictures and descriptions of the different types of overnight accommodation are on www.seat61.com.

There is also a good intro to night trains at http://tinyurl.com/y57vft.

Remember the 7PM rule when you travel with the Flexipass at night. I recommend the 1st class sleeper instead of a couchette. I had a great ride and sleep from Vienna to Frankfurt last year. IMO Frankfurt is not of much interest unless you are there for business.

t

You can also make the reservation in the U.S. thru the agency you bought the pass from.

I have taken both these night trains a few times and never had a problem making reservations in Germany - i usually do at the first train station i come to.

Last Jan took Vienna-Venice Allegro night train and booked a few days earlier in Berlin.

Our first "train" trip was a learning experience - and I can't stress enough the need to make reservations well in advance for couchette's. Intitially, I felt like we missed out on the full 1st class treatment by not pinning-down our travel plans better, but years later I realize that I'd prefer to be more fluid with my plans & we just book ala carte, with minimal advance. You'll usually still be able to travel - in a downgraded passage - but the amenities you paid for (assuming you bought a better class) are only available to space-available reservations. Also, be aware that it is YOUR responsibility to cancel the next day's use on your Eurail pass on an overnight'er. It could result in a fine (or a scolding, which we received) if an unkind fare collector rolls through in the a.m. and you haven't cancelled that day's travel - as simple as putting your initials over the next travel day on the pass. btw - the DB staff were the most helpful travel advisors (aside from this site!) that I've ever had the pleasure of working with, and the hauptbahnhof lounges (again, for 1st class) were top-notch. have fun!

Availability for couchettes i think peaks in the summer travel season, especially on trains between north and south Europe

but in the off-season when i often travel i've only once not been able to make couchette reservations a few days ahead of time and i've literally taken hundreds

One time in Sicily in November wishing to return to Milan via night train all the couchettes were sold out for weeks and i had to pay for a triple - not bad for such a long ride really but twice as much more.

That said it appears fairly easy to reserver via the bahn.de site so if sure of dates do it.

Fridays and Sundays seem more crowded on night trains.