One of the themes of the show is the Rainbow. This may explain why we don't have a Black Ranger.
Their primary mech, ToQ-Oh, is set up differently than previous train mechs. It is set up so that the colourful trains remain continuous, parallel bands... like a rainbow.
Also, the name of the villains revealed so far has to do with the name "Black" in other languages. With the one in Japanese reserved for the Mooks.
Why is the Rainbow the theme of the heroes and Shadows the theme of the villains? It's because they are both created by light and are the exact opposites.
ToQ 6 is notably the first Sixth Ranger since Deka Break whose color is neither gold nor silver. Instead, he's orange, which is one of the colors of the Rainbow.
Not only that, but his name is just one letter away from Tetsu's own name: Aira.
Simply numbering our heroes instead of naming them by their colors makes sense with the reveal of their color-swapping gimmick.
Trains are also often referred to by number.
Genius Bonus too. The color of light that travels the farthest is red and the weapon associated with it in this case is a sword, while the color that travels the shortest is blue and the weapon associated with it is a gun.
It's appropriate to have the villain motif be inspired by Victorian and Prohibition Era fashion, since those times are notorious for societal repression. People were given strict rules on what they could or couldn't do, even so far as telling people what to drink or think. It makes sense our free spirited, childishly imaginative heroes would have such dogmatic tradition be their enemy.
It goes cross-series. Whereas the Tokkyugers' enemies are Victorian and Prohibition Era lawmakers/lawbreakers infamous for societal repression, Gaim'sinitial enemy is a moremoderntake of that same concept.
Grita's love for General Schwarz is also reflective of popular stories of those eras. A young maiden is arranged to be married to a powerful aristocrat but is truly in love with a military officer.
Why is it that when the ToQgers unlock their missing memories, it involves 2 people? Because they're making connections! The unlocking of memories, the color swapping, all of them are train related!
ToQ-Oh Kyoryujin uses the correct naming convention of both series' it represents, i.e. ToQ-Oh[X] and [X]Kyoryujin.
The ToQgers having a crossover special with the orange-themed Kamen Rider Gaim may not only have been to hype up Kamen Rider Taisen, but also as a means of Foreshadowing the later arrival of ToQ 6, the team's orange-colored Sixth Ranger.
In #9, when Kagura passes her Pink Ressha to Mio, she is letting her be The Chick of the team, the role Pink Rangers are usually in among the Five-Man Band, which makes sense with the theme of roles being switchable.
The Build Ressha has a similar formation to the Kuroliners when it becomes Build-Dai-Oh, which makes sense given Akira's past
Akira's guilt over what seems to be a small crime (ruining sunny days with rain) as a Shadow Kaijin makes a great deal more sense when one considers that the emotions that caused still produced darkness, which contributed to the Shadow Line's plans and (as they were building it up overtime) helped allow the Darkness Emperor to arrive. So while it SEEMS like a small thing, it actually makes sense he feels guilt over his actions.
Akira's clothes besides his hat, jacket, and pants are black colored, similar to how the other Shadow Line members are dark clad.
Why does Akira like bathouses? It's likely more that he doesn't like showers - after all, he caused constant rain SHOWERS as a Shadow... The falling water may bring back bad memories.
He's technically a water-based creature, getting to soak in his element should prove beneficial in more ways than one. It actually fits the whole idea of "rejuvenative" baths the Japanese always sell.
Akira being a Western-themed Shadow makes a lot of sense when you consider that Western shows usually have train robbers, people who stole from big lines like the Shadow Line.
Mio's paper lantern wish for her dad not to get hurt makes sense when you remember that she went and joined the police when four of them forgot they were ToQGers. Her dad's a cop too!
Mio's kimono in #42 is predominantly colored yellow and green. While this might seem like she'd end up with Hikari, does anyone want to bet that it's relating to the mixing of her and Tokatti?
In the final episode, Right was able to finally transform into Tokkyu 6 after the first attempt failed. When Akira first joined the team, he didn't know the other Rangers, who in contrast were childhood friends. Now that they've fully bonded, they were finally able to swap colors.
With the events of the movie, we now know that Akira is basically their Imaginary Friend.
A brilliant behind-the-scenes one for ToQger Returns: setting the portion with the kid ToQgers 20 Minutes into the Future in 2017 not only avoids conflicting with the team-up with Ninningers six months later, but also leaves them free for a possible appearance in any 40th anniversary episode or movie in Zyuohger (this was written the December before Zyuohger and is just speculation that Toei was just thinking ahead they wanted something with the ToQgers).
A meta one from after the revelation of Uchu Sentai Kyuranger (and likely not intentional on the part of the relevant producers): after Kyoryuger producer Takahito Ohmori said "Yellow makes a male ranger seem feminine", three of the following Sentais (Ninninger, Zyuohger, and Kyuranger) had male Yellow Rangers, seeming to defy Takahito Ohmori and also making ToQger the odd series out with Mio/ToQ 3. That is, until you remember that not only is Mio the Tom Boy to Kagura's Girly Girl, but her designation is ToQ 3 and not "TQ-Yellow" because the ToQgers can and often did change colors during their series. ToQger was a baby step of reverting away from the notion that "Yellow = feminine".
In the Gaim vs ToQger special, Right follows Kouta into the Helheim Forest. He sees one of the fruits and almost gets ready to eat one. If it weren't for Kouta intervening, Right would have become an Inves with very little hope of turning back to human.
Akira referencing his death could be from the fact that being a Shadow, he wouldn't live long in the bright outdoors......
Meta Example: Episode 46 didn't air as scheduled... due to news coverage about a journalist who was beheaded by terrorists. A lot of kids will have seen that news report when they expecting to see colourful trains...