You see that pic? That's a very low power ion engine shooting Xenon out of its back. It generates less than a tenth of a N of power. SC ships generate tens of millions of times that. Even the smaller thrusters do. An updated version is the VASIMR plasma engine, and guess what? it also leaves a trail, and it's MUCH brighter. So, why do these trails exist? What you see is an atmo test. Contrary to what you may think, this makes the trail mich fainter. In space, the charged gas (AKA, Plasma https://simple.wikipedia.org/wiki/Plasma_(physics) ) does not have anywhere to send its charge other than to emmit photons to lower its electron orbitals to neutral levels. This means it will glow for a longer period. Mix that with the power and, if anything, the game's engine trails are waaaaay fainter than they should be. So that's it. According to physics, the engine trails in space should be blinding strikes of coloured light, but since CIG people are nice (or maybe the glass in our helmets, cocpits and stations is tinted with a special substance) they are of a pleasant brightness.