Yurakucho�fs Kotsu Kaikan Terrace�fs hours

I�fve been searching for this terrace�fs accessible hours. Found here it�fs only opened until 4:30pm: https://experiencetokyo.net/free-view-terrace-to-watch-shinkansenbulle...
Is this true? I am hoping to visit at 6pm on a Monday before dinner.

Indeed, the official website indicates 16:30 as closing hour:
https://www.kotsukaikan.co.jp/floor_map/3f/#teien

Is your aim to see the trains close? Though not as close, you could try the outdoor garden of KITTE building (on the 6th floor) near Tokyo Station. You can see the Shinkansen trains as well as the red brick Tokyo Station building from there.

For a closer look at Tohoku Shinkansen trains, some websites recommend going to Starbucks in SAPIA Tower (also near Tokyo station).