Depending on the Jurisdiction, many of the former border guards were integrated into a national police responsible for internal security. In Germany, where state police are responsible for local law enforcement, the Federal Police are responsible for railways since they cross the state borders.
These forces, togeather with Customs and state police, have always remained active in one form or another behind the border. Customs may check anyone within a 30 KM distance from a customs border, checking for goods allowed in one country but not allowed in another. Between Germany and Poland this often occurs for checks against weapons, fireworks or fertilizers - where different standards exist.
Simple barriers are placed on road or bridge crossings:
- river/mountain borders are easy to control (Germany / Poland, polish side)
- City bridge Frankfurt(Oder) / Słubice, Polish side, 2020-04-06
- Land border (Germany / Denmark), less so
There are still national and customs borders depite the European Union, only their tasks are now much more simplified than they were before. In most cases most people don't notice it (which is the intention), but the infrastructure is still there, but have been scaled down.
Such measures cause a strain on such resources, but it is intended only for a short term and the amount if people out and about are much less than normal.
Well, the German-Danish land border has recently been equipped with a fence against migrant boars ...
This fence (1.5 meters high) was built between January and December 2019.
You will also find such (lower) fences on the German / Polish border, where the river in not very wide.
- Fürst Pückler Park, Bad Muskau, Sachsen, August 2019
Also in Berlin it is also not uncommon on the river front to forests, where until the opening of the border, the the border fortifications were an effective barrier (Western Berlin was until then rabies free).