Estonia - Latvia
In early 2020 I visited the border between Estonia and Latvia and photographed a few border markers, although taking pictures of border markers wasn't my hobby back then.
I returned in July 2024, again during a train journey with a long stopover.
The border markers probably date from the early 1990's, after Estonia and Latvia became independent countries again.
The markers are black and white poles with a "block" on top of it.
A number is clearly present on the markers.

The markers are numbered from east (tripoint with Russia) towards the west (Baltic Sea).
The first marker I pictured from the east has number "201 +257".
I visited this location already in February 2020.
The next maker is 105 meters away and therefore has the number 201 +362. It is not far to border marker 202 and a few meters futher is 202-1. At marker 202-1 the border makes an angle of 90 degrees.
For the next part the small waterway Varžupīte or Konnaoja makes up the border.
I think there are two more markers between 202-1 and 202 A, but I did not visit them.
The next marker is at the Semināra iela/Sepa. This used to be a border station for cross-border traffic.
Since my last visit in 2020 a mini golf has new opened next to the old border station.
It is completely on Latvian territory, but all holes are border-themed.

Accros the street is marker 202 A1. I photographed this already in 2020. The sign on the bridge is new.
The border area has transformed more since my visit in 2020. There is now a path next to the stream, towards the next markers. The border still follows the stream here. The marker is within Latvia. The bridge at the new border marker is new as well. There was an old bridge and less-friendly border crossing before, as google street view from 2011 might show you.
Now it is a spacious pedestrian road crossing the border, surronded by a lot of flags and markings.
A bit more to the north is this swing. It is directly on the border, so you can keep swinging from Latvia to Estonia and back. This is also an addition since my last visit in 2020. There are two more markers on the Latvian side of the stream, 202 A4 and A5. At marker 203/203-1 the border leaves the stream and makes another 90-degree angle. 49 meters down the road is marker 203 +49.

The last marker is also the first one I ever photographed in Valga, back in 2020.
Marker 204 is where the Rīgas iela crosses the border. If you follow it into Latvia you will ultimately reach Rīga.
There has been a border office in the past as well, but nowadays cars can pass freely.