From N3V's latest post: "...recently introduced memory management system..." and further: "...the limiter...". So there is a memory limit in SP6 if someone still was in doubt...
They did communicate via the Newsletter touting SP6's performance optimization. But they never play their own game, otherwise they would know that SP6 looks like an amateur game in an early development phase.
That's exactly what I experienced (reported here) and N3V's answer is here. It was in the RC build but same issue once SP6 was pushed on. No further news since... Note that several other routes (including DLCs) have the exact same problem.
126273 is TRS22 SP4, so you can patch your game to SP5 (128487) then to SP5 Hotfix (129335). SP4 is Trainz Build 5.5, so everything 5.6 will be incompatible if you don't patch to SP5.
From dozens JR's payware locomotives, and "Eagle River Railway" TRS19 & "Coal Country" v2.0 payware routes; also from NARM's "GILA Railway"; and from "Model Trainz: New Haven Industrial Park" & "Santa Fe Needles District" DLC routes.