That's a complex question for me. Early and modern are subjective. They could mean Civil War teakettles vs. Big Boys, or they could describe the twentieth century motive power evolution of one railroad.
As should be no surprise, I primarily like Santa Fe steam engines, I like the ATSF steam that was in service from 1940-1957, which not only includes the "big three" most modern engines on the system (4-6-4's, 4-8-4's, and 2-10-4's built between 1937 and 1944) but also the hard-working 2-8-2, 2-8-4, 4-6-2, 4-6-4, and 2-10-2 engines which date from 1916 through the 1920's. All of those engines were the "modern" appearing engines on Santa Fe, with straight boilers and the headlight mounted on the smokebox door. It might surprise some that I am not fond of 4-6-4 3460 ("The Blue Goose") or the decked-out Valley Flyer pair of old 4-6-2's, as I think all three of those engines were esthetic failures.
However, I also have a fondness for the ATSF engines built between 1900 and 1915, some of which continued to serve until the 1950's. These engines had the headlight mounted high, at the top of the smokebox, often had wagon-top boilers with small smokeboxes and larger sections toward the rear, and were of the 2-8-0, 4-4-2, 2-6-2 and 4-6-2 wheel arrangements. To me, these engines had a quaint appearance that disguised the fact that they did most of the work on the secondary, moderate-sized, ATSF freight and passenger trains, especially on the secondary and branch lines. In their original configuration as cross-compounds, they don't hold any interest for me, but, as rebuilt simple engines, the way they served on the post-1930 Santa Fe, they are attractive. The older 2-8-2 and 2-10-2 locomotives with the high headlights and small drivers are interest me only marginally. This is due to my years in engine service on Santa Fe. The old=style 2-8-2 and 2-10-2 engines were disliked by the older men on the roster where I worked, all of whom were steam men. They universally disliked the very rough ride quality and the 30 MPH maximum permissible speed imposed on those engines just did not fit well on the ATSF of the 1940's and '50's, making them last choice for assignments, and they saw lots of time in sidings for meeting and being passed by faster trains. The old slide-valve "soakers" are quaintly interesting, though they never created any particular warmth in my heart.
However, in general, I like at least something about all steam engines of all railroads, for different reasons according to their performance and/or appearance -- even those that were "ugly as a mud fence". On my layout, though, the paragraphs above are my answer.
