Tramadol does not contain serotonin per se.

But it does act on serotonin receptors. This may be a function of the action it has on mu receptors (pain area) and other receptors like the kappa one.

Some opiates also do this. Oxycodone has quite a serotonin connection in some people.

Some neurotransmitter actions are cascades...initiated by one system and then a second one becomes active. Histamine in the brain is like this, initiating other systems.

The bottom line is that many drugs overlap onto other systems and have actions that complicate things for people.
Some of these are discovered during trials, but many take years and years to come forward, during post marketing.