From what I see it is just enabling rendering of a patter, without merging split railway ways.

In my experience (I tested such rendering during road redesign) it leads to a poor results where railway lines are split into many small segments (for example due to bridges or in places where people tagged changing maximum allowed speed on railways).

On this examples it is not as bad as remember (maybe Kraków where it was certainly noticeable has more split railways than other places?), but still noticeable.

For me this problem is small (I would not notice it if you haven't shown it), but gains should be obvious. Split ways are bigger issue for road labels (names and shields are sometimes located on the edges, where the road splits into "connectors"), but even there it's not severe.

Cracow example looks great to me:

Artifacts (short/missing black parts) are also visible but the problem is lesser than I remember.

Maybe I tested it on z11? Or with longer dashes?

Cracow example on z11 looks also good:

This resolves part of #1704

Thanks for reminding, I have completely forgotten about it!... 😄

Here I would want to do it at least for z11, z12 for railway=rail without tunnel=yes.

I've tried to fix the obvious problem here (trams vs. trains), which exists only on z12. However with this small pattern it might work on z11 too, probably with a bit faded color (z11 areas are also a bit muted and there is less other objects, so railways could be too visible then).

The technique is to use ST_LineMerge
This comes with a few issues

• The SQL is more complicated

I'm afraid of that. We can't afford getting rid of SQL for performance reasons, but given that roads code is already hard to maintain and was meant to be cleaned, I don't think it's safe.

• It is slower

It depends how much slower.

• It involves linestrings that differ depending on bounding box, resulting in artifacts at MT edges
  Because we don't label railways, I don't think the MT edge problems matter here.

We already have railway labels, so this should be tested.

From what I see on test images, I don't feel glitches are big enough to try any workarounds. Maybe smaller pattern make them less visible than the pattern used on z12+? My goal was to make it small enough to not dominate everything around, and the result is that the pattern is there, but not anything above it, so probably glitches are also harder to be found.

Cracow example on z11 looks also good:

For me glitches directly below "Kraków" label, on track below and toward right from city label (black parts are almost completely gone) are significant at z11.

There are also other quite significant glitches.

BTW - after your comments I have also found glitches on current rendering, so this looks like general problem.