My general suggestion would be to go for very long lasting pre grouping locos so you can cover a lot of eras with one model. My personal favourites would be a L&Y Radial tank, LNWR coal tank or a Midland Johnson 0-4-4T - suitable for branch line or mainline and any time from 1890s to 1950s. By the same token a long lasting 0-6-0 such as an LNWR Cauliflower, L&Y Aspinall 0-6-0 or Johnson Midland 0-6-0 would span many eras, and were all used on passenger, goods, mainline, branchline, suitable for most layouts. The Johnson locos suggested above would also work for M&GN and S&D variants as well as Midland, LMS and BR. I understand that other regions as well as LMS produced locos which some people find interesting but I have never quite seen it myself...! More seriously, here would be my list, trying to keep it to commercially viable stuff (approx by era): Pre Grouping, long lasting and potentially commerically viable: GER L77 0-6-2T (LNER N7) - nice suburban loco, one is preserved and popular. Would look OK on mainline and branch. GNR Class C1 4-4-2 - could potentially also do LBSCR Atlantic as so similar. Just because they're lovely! Highland 4-4-0 Small Ben - lasted a long time, and could potentially do a lot of similar Drummond 4-4-0s with same tooling (both Drummonds!) and a bit of licence on wheel size. The small inside cylinder 4-4-0 is the typical British loco. L&Y Class 5 Radial 2-4-2T - long lasting, suburban, mainline or branch, some interesting variants to sell more models, one is preserved... LWWR Cauliflower - long lasting all purpose 0-6-0, wide ranging. Round top firebox not Belpaire please! LNWR Coal Tank 0-6-2T - long lasting, used on huge variety of duties, mainline or branch, one is preserved... Midland 4-4-0 Compound - interesting, popular, long lasting and one is preserved. Midland Johnson 0-4-4T - long lasting, suburban, mainline or branch, some interesting variants to sell more models (M&GN and S&D, with a bit of licence), wide range MR Johnson 2F 0-6-0 - long lasting, mainline or branch, wide range. Later stuff likely to be viable (nothing Southern so far): SECR/SR N Class 2-6-0 or SR Class U Class 2-6-0 (could even sell the Irish version!) SR 'Lord Nelson 4-6-0 Rebuilt Bulleid pacific Pre Grouping, just because beautiful and interesting (not seriously expecting these to be viable!) Caledonian 4-4-0 Dunalastair - because why not! Caledonian 0-6-0 - representing many similar School-of-Drummond type 0-6-0s, long lasting and one preserved. Furness 4-6-4T Jumbo (or LBSCR 4-6-4T) - love a Baltic! GCR 11F 4-4-0 Improved Director (LNER D11) - sutiable for Scotland as well as Great Central and one is preseved. GCR 8B 4-4-2 Jersey Lily (LNER C4) - so handsome! GCR 9K 4-4-2T (LNER C13) - nice branch loco NBR Class H 4-4-2 Atlantic (LNER C11) - because I want one! LNWR Improved Precedent Class 2-4-0 N° 790 'Hardwicke' - why not?!?!? Highland 4-6-0 Castle - quite long lasting and very handsome. Highland 4-4-0 Skye Bogie - because I want one! L&Y 4-6-0 Dreadnought - because I want one!