Hey, sorry I don’t get your question. 

Is there a route where you can’t get a reservation? Most of the reservations you can get via Eurail.com, but I would recommend to avoid Eurail.com if possible because it cost 2$ booking fee on top of the reservation fee. 

Here in the community we have an overview of all ways to get reservations for trains. If you have further questions feel free to ask here.

For normal tickets-incl. de enormous amounts of various discounts in whatever way-just check the sites of the companies-though even what not all manage to put all online.

You likely mean ONLY the extra to pay for RES additional to passes-in fact many a large railway (like SNCF for FR and also TrenITfor IT) did offer that in the far past, but have take that away due to rampant fraud-people who only paid for the RES and did not have the proper ticket. This remains an eternal problem on howtodo for most railways. PLUS that -quite other as planes/airlines-here in EUR railways mostly did not have a heritage of need to RES-only since 2000 and the rise of easy to do IT this is becoming more widespread. Plus that many a USAer seems to have terrific fear that trains are ´full´ weeks ahead-which as such is hardly a problem in general.

All in all it mostly boils down to only the hi-speed superfast rains in some countries-and in fact these do operate mor as clones of airlines.

The extra to pay-mostly just a few €-except again for those superfast lines. But it seems more that each&every newbee to come here seems determined to ride only those lines.

Hi, thank you for your advice.  there have been some routes where getting the timetable and reserving through Eurail.com is not available.  And between two different countries and train lines, also does not seem possible on Eurail.com.  Instead, we have looked at reserving directly, say, on Eurostar for London to Paris.  Now, we need to look at Paris to Graz, Austria.  I know that this involves maybe 3 different train services, so if Eurail gives access to reserve from Paris to the end of the line in the east of France, near Geneva for instance, then do we need to reserve on Swiss rail through Switzerland?  And the Austrian line (OBB I believe) through Austria?

If you read angelo’s post he gives a link about getting reservations. Having read it the community are happy to help with further questions. You also need to refer to the Eurail map because you would not normally go to Geneva if trying to get to Graz from Paris!! One of the best apps for route planning is DB just type in Paris - Graz. Once you have an itinerary and read the how to reserve section experts on this site can help if you are still stuck. Internal reservations in Switzerland are not required. In Austria reservations are advised on some trains - if you let us know the train/date we can let you know.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

To answer your fundamental question as simply as possible:

Every country will accept your pass as your travel ticket, but where reservations are possible (both mandatory or optional) the ways of getting those reservations online are confusing but generally fall into 3 categories

1. Easily purchased via sites like OEBB, Dbahn and Czech (Cz), but you need to know how to set each one to give you just a reservation. These sites sometimes offer reservations for other countries ( OEBB and Cz are good examples).
2. Dedicated sites for pass holders  including Interrail/Eurail reservation service - particularly the B-europe sites for TGV, Thalys and Eurostar. 
3. Those countries where it is more difficult to purchase reservations in advance - e,g, Spain. These often involve phone calls, station visits and posted reservations.

All the guidance for specific countries is shown in the links above.

It is worth mentioning that for the majority of services there is little need to book months in advance, with the main exceptions of sleepers and Eurostar. Some routes have high demand at peak times but most of these are rarely full and most have several alternatives not requiring reservations, but may be slower with changes.

If you need advice on booking reservations, then please mention for each train: route, date and departure time. Without that information, we can only guess which trains you're trying to book.

Thanks to all for these responses.  They helped greatly.

@Addie M as rvdborgt said 

If you provide like Dates and Routes we can provide more detailed advices :) 

e.g. the most direct Route from Paris to Graz would be by train via Stuttgart or via Zürich :)