Poland in 1918/9 inherited three different track widths (german,austrian and russian)

Both Germany and Austria had standard gauge, maybe you mean narrow gauge railways?

Regarding compatibility, this is the main problem for international traffic. Track-related issues aren't even the worst of it. Issues include:

1. signalling system
   
2. electric system (AC, DC, which voltage and frequency, what voltage stability)
   
3. maximum cross section
   
4. track width (gauge)
   
5. maximum axleload
   
6. rules of operation
   
7. train staff education
   
8. couplers

The situation on each of these:

1. Systems are essentially different from railway to railway (sometimes even within a country). Nowadays border-corssing vehicles are fitted with multiple systems. The EU pushes the ERTMS (levels 1-2-3) to standardise this, but it is a herculean task, and one plagued by technological problems. There are only half-functional pilot lines with ERTMS.
   
2. There are four main systems: 1.5 kV and 3 kV DC (in BeNeLux, half of France, Italy, Poland, half of former Czechoslovakia and ex-Yugoslavia), industrial voltage: AC 50 Hz, 25 kV (rest of France and the other 'half DC' countries, Denmark, Hungary, all new high-speed lines in DC regions), and the 16.7 Hz / 15 kV railway AC (German-speaking countries, Sweden). Expect AC to slowly take over some DC lines, but overall, this separation will stay. However, modern power electronics made multi-system locomotives an econmic option.
   
3. Cross sections were internationally standardised more than a century ago. However, there are too many of them with different sizes, posing different limitations. Generally, one could say that Britain has a very narrow cross section, France and BeNeLux and Switzerland and Italy the next smallest, other Germanic countries and Hungary have a somewhat wider but significantly higher one, while the Iberian peninsula and Scandinavia has a significantly wider one. But, non-national differences are in corner height: some railways have room  4m high for truck- or large container-transporting cars, some don't. But for future high-speed lines and trains, the narrow French norm was adopted, so no differences here between the newest Spanish, Italian, German trains or the British CTRL line.
   
4. Russian broad gauge rules in the ex-Soviet-Union and in Finland. There is one broad-gauge line deep into Poland ending at the Czech border. The even wider Irish and Iberian gauges rule in the respective countries. However, Spain has a very ambitious program to rebuild all of its (surviving) broad-gauge lines to normal gauge. This started with building high-speed lines in normal gauge and upgrades with sleepers that have jacks in two positions for easy re-mounting of one rail. There is one lesser known, somewhat related difference between railways: the lateral inclination of the rails (1:20 and 1:40 in different countries). How much effect that has is still subject to research.
   
5. The main difference here is between branchlines and mainlines, but across Europe, there is variation in the latter, too. On most mainlines, it's between 17 and 22.5 tons per axle. There is a push both from the EU and some railways to raise it on main freight corridors to 25 tons, but to do it across the whole European network would be very expensive. On the other hand, in the main factor in making a track suitable for some axle-load, rail type, there is a well-progressing development towards using UIC-60 rails (means: 60 kg/metre rails according to UIC norm). For high-speed trains, following the French-Belgian norms, 17 tons was agreed, though note that tracks have to bear more due to aerodynamic load.
   
6. Some rules of operation are international, some related to the railway line traversed, some to the signalling system, some unique to railway company. The EU packages include standardisation or provision of information or instruction about differing systems to operators.
   
7. Standardisation here is included in the EU's Third Railway Package.
   
8. The former Soviet Union has its own automatic coupler, the rest of Europe a standard non-automatic coupler. Normal-gauge railways attempted to introduce a standardised automatic coupler in the seventies-eighties, but it foundered upon sabotage by cash-strapped railways (chiefly East Bloc). But from the nineties, a family of less powerful, lighter automatic couplers (the Scharfenberg type) became almost standard on new-built multiple units.

The sensible part of the EU Railway Packages pushes changes to increase compatibility on many of the above detailed fronts. The rules define a truckload of so-called TSI, Technical Standards of Interoperability. I also note that on another front of standardisation, rules for testing and accepting new railway vehicles, international railway organisations already progressed much without the EU, though EU rules are needed to break down the practice of requiring tests on the same subject for the same vegicle in each country separately.

_{*Lunatic*, n.
One whose delusions are out of fashion.}