The latest new ones (of which I am aware) are in ASEAN=South East Asia and have succeeded in linking Laos-previously even without those things on rails:
the first to open was by the Thai SRT-a short spur from NongKhai, its former terminus, on the Maekong river, via a bridge to a far-away suburb of the capital Vientiane. Thats just 2 short shuttle trains/day-but recently-or about to- lengthened furhter into but still suburban VTE.
On its north side Laos has sold its soul to the Chinese and these have in the usual record time built a -single track!-hi-speed line from their border as far as VTE again-but somewhere in suburban north side=opposite from the Thai side. It starts from Kunming in Yunnan.
The THai have also done more for their part of the work and have made the extension of its eastern line -ending at the small town of AranyaPrathet to the border with Cambodia. But the KHmer are not that fast and have even shortened the very few services over rails in old state from Pnom Penh-they never have reached that border from their side.
But there is a casualty too: on its south side the former 1/day through sleeper train, much liked by both bekpek-tourists and rail-aficionados-from Bangkok to the opposite side of Penang island (Butterworth) now ends at exactly the border station-the Malays have upgraded and electrified their part-there is still a fairly easy connection
JUst a day or 2 ago read about a possible re-instate of a direct China-Kazakstan link, suspended since covid and apparently now revived, but only for large prebooked groups around festivals or so
Direct China-Mongolia trains also are said to revive soon-or may have done already, but not (yet?) the all the large way Trans-Sib from Mockba.
In AFrica there are talks/plans to revive the direct train link TUNisia-ALgeria (May perhaps even been posted here as item a week or 2 ago)