Agree'd Larry. My last basement layout was designed with 18.5 foot passing siding capacity (no layout currently). If I were able to keep that length in future layouts, that would allow me to run 25 car coal trains with the Trall cars and have 3 diesels and fit in the siding. A 25 car coal train on a home layout with 30-inch curves looks pretty long.
To answer the question posted about length, a 30 car train without engines would be about 18.5-19 feet approximately. (I'm allowing about 7.5 inches per car coupler to coupler (but I'm at work and can't measure them). Add engines, probably 3, and a caboose and you have a 21-22 foot long train - thats getting into the club size range as Larry suggested. Some home layouts can manage trains like that... and keep in mind that if most trains fit in sidings, you can have an occasional train which is longer than the siding and pass the other trains that fit. So having one or two overly long trains is possible as long as you can break it up or provide a track or two in staging for oversize trains. My last basement layout had 10 tracks of staging and using curved #8 turnouts (Shinohara) I was able to squeeze out siding length to between 18 feet minimum and 24 feet maximum. So I could have a couple extra long trains. Also the yard I designed had at a longer set of tracks that included part of the yard lead to handle trains up to about 24 feet as well. So passing track capacity is one major consideration for train length but not the only one.
As for being slow and missing out on stuff for sale, yeah it makes it more difficult. Certain things don't tend to show up on Ebay cheap. For example the Walthers Rio Grande 100 ton 4-bay quads were issued in set #1, #2, and #3 over a couple years. I did buy them as they were issued of course, but after a few years they were long gone and when the rare set did show up on Ebay, they would get bid up to about double the retail price! I don't think it will be that way with the Thralls, but most of the deeper discounters will be long sold out of the road names some of us want. My pusher offered the Athearn Tralls at about $66 per 5-pack ($13 per car). I paid that for the first 4 sets and then UP removed the surcharge on UP and D&RGW cars, and the price went down to $55 per set for the remaing four so far.
Speaking of the Walthers quads, during the recent years that they were hard to find and high in demand, a guy on our Rio Grande email list offered up 2 sets assembled of the quad hoppers for $80 shipped! This was during the time people were paying $110 or $120 regularly for the 6-car sets in kit form on Ebay! Needless to say I jumped right on it and emailed him. I have 36 of those quads at present but probably should have more as those were by far the most coal cars owned by Rio Grande in terms of quantitiy - approaching 2000 cars.
As far as missing out - I kind of have missed the boat partly on the CSDU cars. Most places including the distributor (Horizon) have only set #3. I located set #1 at Caboose Hobbies for $62 which isn't too bad. Probalby there are a few places with set #2 but I'll have to pay thru the nose unless I find a discount seller that is obscure.