I found this photo of the blue 409 in use in the late 1950s:
https://www.eurovapor.ch...mp;type=orig&id=2905It shows the loco in use with SBB lightsteel coaches. The markings on these coaches correspond to the 43362/72/82/92/42402.
Maerklin does not, as I have stated, have a pilot coach for that time frame.
As far as fantasy options, I came up with a few possibilities:
1) Rheingold 1928 coaches. This loco would probably match the colors of the classic Rheingold consist
2) CIWL consist (LS Models, Rivarossi, etc)()
3) Rheingold 1962 coaches. This loco would probably match the colours of the Rheingold consist introduced in 1962, prior to inheriting the red/beige TEE colors.
4) Piko or Liliput or Lima NPZ coaches. The white and blue with yellow doors NPZ coaches were many years after the blue period of this loco, but the colours are probably a match
5) Maerklin, Fleischmann or Sachsenmodelle S-Bahn Zurich coaches. Another pure fantasy in similar colors.
6) City Night Line (CNL) coaches from Heris, LS Models, ACME, etc (Once again, just because of the colors)
7) NightJet (NJ) coaches by LS Models/ACME/Roco (Similar colors)
Your idea about using TEN Sleepers, Eurofima couchette (4168/4268) and even maybe the NS 42696 couchette also works based on the colors.
As far as Stefan's comment, I would think that by the late 1950s, there probably were questions as to whether railways needed to keep the olive green colours or whether they could explore other colours. In Germany, the blue/beige livery for the prestige Rheingold and red/beige for TEE trains throughout Europe, the Treno Azzuro in Italy and others are examples.
Whatever you choose, as long as you like it, is your option. We used to have a 3013 NS 1100 with CIWL coaches (1950s Maerklin) and called it the Blue Train when were were young.
That was my preferred color but the green train RET 800/3014 plus the coach with opening doors was so much cooler!) I later found out that it was Swiss and that reinforced my attraction to Swiss trains. My youngest brother had no problem adopting the blue one as his train.
Regards
Mike C