Fixed it. The <play> was running just fine. The difficulty arise from the builder method wich doesn't automatically add up the <key_frame> in the <do ... end> block. Instead, I used the clearer java counterpart : 

a = Vertice.new
b = a.x_property
b.add_change_listener {|new| puts "x trace : #{new}" }
c = timeline(key_frame(1.sec, key_value(b, 100, Java::javafx::animation::Interpolator::EASE_BOTH)))
c.play

... which runs just fine. I couldn't get the writing of the constant <EASE_BOTH> as a symbol working. The last argument can be ommited.