That would be a good way to narrow down the list. For each place ask yourself "why?"

If you don't have a reason... maybe it doesn't make the cut for this trip.

The efficacy of a railpass is if the person wants flexibility as I and several others here suggest on such a long trip.

Discounted tickets must be booked in stone and cannot be changed nor refunded typically and are sold in limited numbers so must be booked weeks/months before.

And it also depends on how many long-distance trains are taken. If a lot like preliminary OP statement suggests a pass could even be better than discounted tickets.

But first OP has to come up with a tentative real itinerary than we can comment better.

Well the OP hasn't been back so this might be a troll - they do love to get fodorites going by posting apparently crazy itineraries. But assuming the OP is for real I actually think an abbreviated version of this itinerary is not that bad.

Barcelona - 5 nights - train to
Marseille (or other town in that region) 5 nights with day trips to Aix, Avignon, etc. - train to
Nice - 5 nights with day trips along the coast (could include Monaco/Monte Carlo) - fly to
Paris 5 nights - fly to
Rome 5 nights - train to
Florence 5 nights with day trips to Siena, Pisa/Lucca, etc. - train to
Venice 5 nights (possible day trip to some town in the Veneto such as Padua or Verona) - train to
Milan 5 nights with day trips to Lake Como, Bergamo, Parma, Pavia, etc. - train to
Switzerland - 5 nights base in Interlaken with day trips into the Bernese Oberland and on the lake, to Bern - train to
Munich - 5 nights with day trips to a Bavarian castle, Bamberg, Regensberg - train to
Vienna - 5 nights - fly to Greece
Greece 10 nights - start with Santorini, ferry to Naxos, to Paros, to Athens - fly to
Copenhagen - 5 nights - train to
Amsterdam - 5 nights with day trips to Delft, etc. train to
Ghent, Belgium - 5 nights, use as base for Antwerpt, Brugge, Brussels - train to
London - 9 nights with day trips

This is 89 nights. Obviously some places maybe more than 5 , others less but in general five nights gets you four useful days which is enough. Fairly fast paced but not crazy. A base for five nights with day trips gets you more useful time in most cases and is a lot less stressful than a whole lot of 1,2,or 3 nights. This trip would still give a really nice overview of Europe - obviously nothing in depth but lots of people go to any of these destinations for less than 5 days and no one says that's crazy. It leaves off the three 'maybes' and also Dublin. Would be possible to shorten London and end in Dublin if that's a priority for the OP.

Some of the cities listed sound like you are just naming large cities you have heard of (Frankfurt, Cologne, Manchester - none of these would be high on my list of interesting places for a tourist to go). So I substituted places that I (and most guide books) consider more interesting from a tourist point of view. Obviously if there is something specific in any of these places you'd have to do some switching. (Though obviously no way you can go to Belgrade while in France - this is what is leading me to wonder if this post is sincere).

It involves 4 flights and 11 train rides (plus day trips). A lot of those train trips will be under €100 and considering the need for reservations on some it doesn't look like a rail pass would save much if anything. But maybe with all the day trips it might.

Budget has not been mentioned at all. The transportation (flights and trains between places, not including getting to Europe in the first place) could easily get to €2000. Plus a minimum of €100 a day for an inexpensive lodging and food and entrance fees to just a few places would be another €8900. So with airfare to Europe included (not sure where you are from) I think if you have €12,000 this trip is doable.

are some of those places one day visits? and you list countries? well yes - every country is IMO worth more than a day or maybe two. Once again look up travel schedules and times and see what logically can even be done.

I also do not have a problem with a city oriented tour of Europe. I think my first trip included Brussels, Amsterdam, Copenhagen, Oslo, Stockholm, Munich, Zurich, Vienna, Venice, Rome, Barcelona, Madrid and Paris for 2 months. This did not include the UK as I was studying in England at the time. We set up a general route before leaving London and allocated time on the fly by leaving each destination when we felt ready. Cost was also a factor so we did Oslo and Stockholm as one day visits with sleeping on an overnight train in between them. Also could not afford staying in Munich during the Octoberfest so spent one night sleeping on the train to/from Hamberg. Do not recommend anyone else doing these types of cost saving measures. Having a Eurailpass does give one a lot of flexibility and we found ourselves changing a lot of plans as we went. Skipped many possible destinations due to lack of time. I do agree totally on the need for breaks. I looked at my step monitor when I was in Berlin a few years ago an saw 30,000 steps for the day and such walking is quite common for us in a city. You can only travel like this for about 2 weeks before you need time to veg out, do laundry and rest. This is particularly true if you also going out at night after a long day of exploring. A Railpass for such a long trip represents freedom. It may not save you much if at all if you minimize your moves and do point to point and I am sure that if you buy your train tickets well in advance that it could cheaper. My opinion is that freedom is worth the premium. Twice in my travels I have taken the train north with the intention of exploring the Norway coast beginning with a train ride between Oslo and Bergen and both times I met a girl and ended upon Gothenburg and Copenhagen instead. Another time I met some guys from Montreal in Nice and we pooled our resources, rented a car together and explored parts of Provence for a few days. I could go on and on but many of my most memorable moments were not planned. My cost guesstimate for a trip like this would be about $12,000 if you do not shop and eat mostly out of supermarkets.

As I said above, many trains require reservations, which also have to be booked for a specific train, and there goes your flexibility.

Be sure you understand the Schengen zone visa requirements. You are limited to a max of 90 days out of any previous 180 days. It is 90 days and not three months. So far with your schedule you are OK since G Britain is not a part of the Schengen zone so the time in England would not count towards your allowed 90 days. However, if for some reason you skipped Great Britain then you could have a problem. And do make sure you get good stamp for entry and exit from the Schengen zone.

Many trains is in fact a relatively few trains considering Europe as a whole - mainly in Spain, Italy and France and you can easily do these once in a city not have to do in stone weeks before as with discounted tickets. Most countries still can hop on most trains anytme - so many is rather misleading in general. So yes a railpass definitely does afford flexibility to change course, stay longer some place or less, etc. 'there goes your flexibility" (with a pass) is simply terribly misleading.

I guess you are all talking to each other now rather than to the absent OP?

Well if OP is around or comes back...

Some first time posters never find there way back...

Now they mostly do if they are actually interested/serious since they get notification of activity (unless they turn the feature off, which most newbies wouldn't know how)

But not everyone would register with their real every-day email address. I certainly didn't.

Who is to say OP is not monitoring several threads of several forums -just because don't make cameo appearance does not mean out to lunch.

Who is to say...

No one said anyone was out to lunch. Just wondering what the OP thought of the ideas being given is all.

poor wording on my part about 'out to lunch' - just meant away for a while like at lunch. Sorry.

a l o n g l o n g lunch!