I figured it must have been green for go and red for stop. If you wanted something stronger there was always some guy selling other drinks in the restaurant car. A useful tip I quickly learned is to put or a slice of bread into the soup. An alternative to meals on the train is to get off the train at each stop and buy food from the station kiosks and other vendors on the platforms. This can be really good but beware getting ripped off on some goods.
Common sense is most useful.
I saw one guy without any who got a 'good deal' on what to me looked like an iffy bottle of champagne. The moral of this is to check the wrappings and seals, etc. Best buy in drinks can usually be from the stewards on the train. Other things to be careful of are your own possessions, particularly on the train where foreigners are seen as easy targets with stuff worth lifting. Petty theft of anything you leave unattended is quite common, again common sense is your best precaution, but then again this is true anywhere.