How to take the truly epic rail journey from Frankfurt to Zagreb

What had been the direct daytime train between Frankfurt (Main) and Zagreb has been suspended until further notice.
The EC train which typically departs Frankfurt (Main) hbf at around 08:20am used to convey through coaches which were detached at Villach, prior to travelling on to Zagreb
However, this train now commences its journey in Villach and as a 10 minute connection in Villach with the train from Frankfurt (Main).
So ShowMeTheJourney is assuming that this connection will be guaranteed, as it will be fairly pointless for the train on to Zagreb to depart on time, if this train from Salzburg is running late.
The connection in Villach should be arranged, so that it's a simple as can be cross platform transfer between trains.

Book early and save: Yes

Online bookings usually open: up to 3 months ahead of the travel date

A choice of tickets

Three types of tickets are available:
Two types of discounted tickets, which will be cheaper the further ahead you book - Sparpreis and Super Sparpreis
One type of non-discounted ticket - Flexpreis,

A key difference between these types of tickets is refunds:

• Super Sparpreis tickets are cheapest, but they can't be refunded at all.
• Sparpreis tickets can be refunded for a fee, but the refund will be in the form of travel vouchers to be used for alternative / future journeys.
• Flexpreis tickets can be refunded at no additional charge.

The other key difference is that both types of Sparpreis tickets are only valid on the specific departures selected when booking, but Flexpreis tickets live up to their name, as they can be used on any departure on your travel date.

Note that none of the tickets can be exchanged to a different departure.
There's no need for this to be available for Flexpreis tickets, as they aren't train departure specific.
If you book a Sparpreis ticket and want to travel by a different departure you can refund it and then use the travel voucher towards the cost of the new booking.
Super Sparpreis tickets can't be exchanged or refunded.

The Sparpreis and Flexpreis tickets also include 'City tickets' which can be used on public transport between locations in the city centre and Frankfurt (Main) hbf.

Seat reservations

Sat reservations are mandatory on the train on from Villach to Zagreb, so they should be included when booking tickets.

Seat reservations are optional on the train to Villach- take note if you will be using a rail pass.
They cost €4.90 when travelling in 2nd Class and €5.90 when travelling in 1st class.
Specific seats can be selected from seating plans - you don't have to accept the seat(s) that you will be automatically offered.

They are included within the cost of booking 1st class Flexpreis tickets.
When booking Super Sparpreis, Sparpreis and 2nd class Flexpreis tickets, you have the option of purchasing seat reservations later.
It can be worth re-looking up a journey a week or two ahead of travel, because the DB website will indicate how busy a departure will be.