Probably 1959 or 1960.
For those of you who do not know, the N & W Y6b 2-8-8-2 locomotives could out pull and out start a Big Boy. Moreover, being Mallets (compounds), they could do it with less coal and water, albeit at lower speed.
Sometime, around dusk in the late 1950s, while passing through Cincinnati with my Father in his 1951 Buick Super, I remember passing beneath the approach to a bridge across the Ohio River on US 50 on our way to US 52 eastbound to Hunnington, VA. We passed directly beneath a huge steam locomotive slowly pulling a freight train South toward the Ohio River bridge.
Probably around 10 at the time, I had already become a RR buff, having grown up on a small farm very near the B & O RR and less than 1/2 mile from where the steam helper locomotive was stationed to assist the westbound DE freight trains up out of the Ohio River valley, westbound to St. Louis.
The helper had been taken off late in late 1956 or early 1957 and it had been a year or so, a lifetime for a 9 or 10 year-old, since I had seen a steam locomotive.
I remember marveling at the size of the locomotive passing slowly overhead at a time when steam locomotives had already vanished for a "lifetime" for one of my age. More than likely, it was a N & W Y6a or Y6b.
Memories, memories.