I'm a frequent visitor to Luxembourg and user of their public transportation system. I agree with most of what is said here, with the following observations.
CFL frontline Staff in the Gare Central are flawlessly quadrilingual (EN/FR/LU/DE), whether working in the ticket office or information counter. Staff in the shops, restaurant and toilets there will probably speak just French or Luxembourgish. The situation in the country is complicated by the fact that there are a large number of monolingual francophones working in low graded white collar work in the capital and south of the country. CFL's website is primarily written in French, although there is a German and English option (note no Luxembourgish). The Mobiliteit.lu website - which is much better for transport planning - is entirely in French, as is the vdl.lu website, covering transport in the capital. Staff working on the railways as guards are expected to speak French and German perfectly and to acquire a good level of Luxembourgish within a year. Announcements are made usually in French and occasionally in Luxembourgish, with stations announced in both languages (which often have a slightly different pronunciation). Road signs and building signs are almost always in French only. Road names are 95% French, with a smattering of ‘Luxembourgish only’ road names (Bisserwée/Bisserweg in the Grund district of the capital, for instance). There is an increasing tendency to name streets with a French and a Luxembourgish name.
Like with Poland, the older generation will have an animus towards German and all things German, not shared with the younger generation. There is some anger from the political right towards monolingual French speakers not wanting to learn Luxembourgish - you can send your child to a monolingual private French school, if you so wish. There was a petition to parliament to make the country monolingual Luxembourgish (which was defeated!). Guest workers from Portugal (of which there are many!) generally learn French only, so getting a taxi from the airport will usually result in you cracking open your schoolboy/schoolgirl French (or Portuguese!). Generally, if someone grew up and was (publically) schooled in Luxembourg, they will speak 3 languages fluently. I’m lucky enough to speak reasonable French and German, and a few words of Luxembourgish. Luxembourgish, although they’d hate me for saying this, has about an 80% lexical similarity with German, so if they speak slowly, or if I can read it, I can understand most of what they say. My rule of thumb when entering a shop or a restaurant is to say “Moien” upon entering (“Hello”) and if the person answers “Moien” I know they will almost certainly speak German, as they will in all probability, be a Luxembourgish speaker. If they answer “Bonjour”, they are in all probability a monolingual “transfrontalier” from Arlon or Thionville.
The Wikipedia article cited above probably states it the best - "language choice tends to reflect social class. Members of the upper middle and upper classes tend to prefer French, although Luxembourgish may be used to convey a sense of close identification with one's nationality. German tends to be viewed negatively among the upper class, with the assumption that anyone writing in German has a poor mastery of French. Despite this, a minority of members of the upper class do prefer standard German when corresponding with close relatives. Use of German becomes more prominent the lower down the social scale one goes, followed by Luxembourgish, with French tending to be the least popular among the lower classes"