歐洲火車證--應該點買

Yes, you can buy it in Hong Kong. You can go to its website www.eurostar.com (selecting your country as Hong Kong), or you can buy it through local travel agency.
There is no direct train from Paris to Interlaken, you need to change in Basel. Also, there is no direct train from Interlaken to Venice, you need to change in Milan. The fee is as below: Paris to Interlaken 86.30 euro. Interlaken to Lauterbrunnen (with return) 13.20 CHF = 8.14 euro. Interlaken to Venice 110.00 euro. Venice to Milan 40.00 euro. Total 250 euro. If you buy eurail select pass for 3 countries 5 days, it will be 268 euro. So you can single ticket will be a bit better
It's not so dangerous. But of course you have to take care of personality belongings. The most serious crime on the night train is thefting and pick-pocketing only... But if you travel alone, I will recommend you take day train, or by flight directly. Actually, if you travel only a few journey, flight may not be experience than train (sometimes flights will be much cheaper...)

NIGHT TRAIN無太危險(由巴黎來回ITALY小偷多些)，若RAILPASS是1ST CLASS加些錢住房(2-6人房,6人房最平HK100左右)有門鎖&乘務員,用RAILPASS買EURO STAR 有折(不同年齡的折又不同)用RAILPASS是最平的(你去咁多地方計)