Anything that can indeed be liquidated. Trains you can't.
No but assets and property can. And the legal means to liquidate the financial assets of Jews who had been stripped of their citizenship had been promulgated just as this train was leaving Berlin. There may have been some grey area as to whether Jews being deported to Riga had immediately been stripped of their citizenship and if they fell under this new law vis a vis liquidation of assets. Jews on later transports most certainly did have their property liquidated.
As to the use of liquidation = to kill, I think the word would have acquired that meaning but only in relation to referring to Bolsheviks practices. Yet ironically, when the Soviets used the word it would not have meant to physically killed. Hence when Stalin published a pamphlet in 1929 "The Liquidation of the Kulak Class" he did not mean that he was publically announcing the physical killing of Kulaks. This usage was derived by anti-Soviets and anti-Bolsheviks in Western Europe to refer to Soviet practices - ie "they say liquidate but that is just a euphenism for killing".
As such we would expect to see Germans use liquidate in a negative sense to refer to Soviet practices such as in Goebbel's total war speech
The German people, in any event, is unwilling to bow to this danger. Behind the oncoming Soviet divisions we see the Jewish liquidation commandos, and behind them terror, the specter of mass starvation and complete anarchy
But not to see it used in relation to their own policies, as they would differentiate themselves from Bolshevism. It was a new meaning to the word that had been developed in particular regard to Boshevism and Soviet terror.
Again, oddly, the Soviets would probably not have used the word Liquidate = to Kill, at all.