Is there any possiblity of not being able to purchase tickets for same-day travel? Are the trains ever booked?
In general, you shouldn't have problems getting tickets and more importantly reservations for same day travel. However, there are always holidays/festivals that you may encounter where things may be sold out.
Figure out in advance what your alternatives are? A few years ago, I found that bus travel was often times faster since the trains would have to change crew and engines each time you crossed borders - hopefully this has changed now.