Personal experiences:
Local trains between Poland and neighbouring Schengen countries are rarely checked. I've made quite a few journeys between stations such as Zgorzelec and Gorlitz, or the very odd Jeseník-Krnov line in the Czech Republic that transits Poland and stops at Głuchołazy-Głowne (but not at the train station in the town of Głuchołazy itself, Głuchołazy-Miasto!) and I've never seen any controls. On the other hand, long distance trains attract quite a bit of attention - for instance, the Berlin-Warszawa-Express normally has border guards looking through the train. Even with the migrant crisis, attention was focused on cars and long distance transport, not local transport.
Road crossings are often monitored, though random controls are made. It depends on the location as to how these controls look - customs officers tend to be more present in places such as Cieszyn-Cesky Tesin because of Poles buying booze in Cesky Tesin, but on the other hand, buses from the Czech Republic towards Ukraine tend to be stopped at crossings such as at Kudowa-Słone/Nachod.
I am interested in how they managed to patrol borders such as these before Schengen (especially in the rural areas where all sorts of small roads go across)
I'll talk about the Polish-Czech border because it's the one that I know best:
Before 1990/1, the border was heavily guarded. It wasn't comparable to the Iron Curtain, but barbed wire fences existed in many places, and there were considerable resources dedicated to making sure that it wasn't crossed illegally. There were small border crossings intended for use by those with passes, but in general, it would have been difficult to cross the border illegally, even in areas where the border cut across villages.
In 1990/1991, things were considerably loosened up. Many tourist border crossings were created, the barbed wire fences were removed in some places (but not all!), and many crossings were opened to most Europeans and a few others (especially Americans/Australians/Japanese citizens). At this point, because of these tourist crossings, controls tended to be random rather than systematic. Vehicle access was still very difficult as barriers were in place, but despite these crossings having official opening hours (often 06:00-20:00 in summer and 08:00-18:00 in winter), little attention was paid to people crossing illegally outside of those times if they were local and not smuggling.
At this time, many informal crossings opened up. While agreement was often lacking at an inter-governmental level, there was often quiet agreements that such crossings could be used, especially if there were services on one side of the border and not on the other (for example, a pub or a shop).
When Poland and the Czech Republic joined the EU, things were even more informal. Many tourist crossings simply stopped being controlled, or were controlled only at certain times (such as on Saturdays), while most people would come and go freely. I crossed many tourist crossings illegally at night and without consequence, and the only time I was ever stopped resulted in a few questions about where I was going, but nothing else.
A good example is
here: this crossing was fenced and closed off before Poland and the Czech Republic joined the Schengen agreement. The only access to the Czech pub shown here was through
this border crossing, and there was also a shop on the Czech side that was notorious for selling large amounts of non-vodka spirits and chocolate to Polish travellers, both of which were cheaper in the Czech Republic before both countries joined the EU. The crossing itself was rarely manned after 2004, but even before, it often depended on how busy the main crossing was in Kudowa-Słone/Nachod.
Main crossings were always guarded, and it was a bad idea to drive across the border, but people on foot/by bike/on skis were rarely bothered. I remember missing the closing time of the border one year in Czermna because I got drunk in the pub there, and this was before the EU came. I asked the landlord of the pub what to do, and he just laughed, because the border had no practical significance to locals.
