Paper:
We propose Symplectic Recurrent Neural Networks (SRNNs) as learning algorithms that capture the dynamics of physical systems from observed trajectories. An SRNN models the Hamiltonian function of the system by a neural network and furthermore leverages symplectic integration, multiplestep training and initial state optimization to address the challenging numerical issues associated with Hamiltonian systems. We show SRNNs succeed reliably on complex and noisy Hamiltonian systems. We also show how to augment the SRNN integration scheme in order to handle stiff dynamical systems such as bouncing billiards.

Review:
This paper proposes to represent a Hamiltonian model of a physical system by a neural network. The parameters are then adjusted, so that the observations are considered maximally likely under a probabilistic model. The novelty is to consider a symplectic Leapfrog integration scheme for the Hamiltonian system, which is known to conserve important quantities such as volume in the state space. The proposed approach is shown to outperform the recent work "Hamiltonian Neural Networks" by a large margin on mass-spring chain dynamics and three body systems. The approach can even handle stiff dynamical systems such as bouncing billiards. 
Overall, the work is solid contribution and a reasonable improvement over the recent work on HNNs is demonstrated. Therefore I recommend acceptance of the paper. However, I have some fundamental doubts on the motivation on this line of works. This might be, because I'm not too familiar with the subject, and I'm willing to increase my score if the doubts are cleared.
In the shown examples, to best of my knowledge, the "exact" Hamiltonians describing the physics of the system are well-known. Therefore, I'm unsure what is the advantage of trying to learn physical laws, that are already well understood. The paper argues that the learned Hamiltonian will correct for errors in the discretization, but one could instead use a better integration scheme or a finer time-discretization, based on classical theory which has been developed over the last 50 years which comes with strong convergence guarantees and error bounds. I would have liked to see a stronger motivation, why it is interesting to learn an Hamiltonian of a system, where the exact Hamiltonian is already known. It would also be enlightening to see some plots, which illustrate how "far" the learned Hamiltonian is from the analytical one.
Of course, one might argue that the ultimate goal is to have a learning based approach discover physical laws so far unknown to humans,  just from observations. But it is unclear why the inductive bias that the observations are generated by a Hamiltonian might be reasonable. It could very well be, that the law cannot be described by a Hamiltonian system. 
From a high-level point of view, one might even argue that it is not too surprising that one can fit a parametrized Hamiltonian to observations generated by a Hamiltonian system better than a general purpose function approximator without such an inductive bias or better than a system based on a naive/unsuitable non-conservative integrator.
As a remark, often the exact Hamiltonian is known to be (strictly) convex. I'm wondering whether convex function approximators such as convex neural networks could provide an even stronger inductive bias. But it might be that a general purpose RNN can account better for the discretization errors.