For a single Mic the best system would be what is known as a 50/100v line system driven from a master battery powered mixer/amp where the microphone is needed, separate volume controls can also be provided in each carriage if necessary. The amp would need to be powered from a car battery or similar.
Dependant on the volume required and how many speakers are wanted, if say there was 4 speakers per carriage then you would be looking at about 1-2w max per speaker (it might not sound a lot but for speech should be more than enough) and therefore for a 5 carriage consist you would need about 40W with some extra wriggle room although the actual power output is not too important so long as it's higher than the rating of all the speakers added together.
With 50/100v line systems the power output of each speaker is set on the speaker itself and therefore sets the max volume of each speaker (well how much roughly it is allowed to draw from the amplifier) and you can get inline variable attenuators (volume control) that would allow you to regulate the volume in each carriage.
As for equipment Thomann.de would be a place to start and this is where my suggestions of possible equipment will come from (this will be equipment that I have had no experience with so will literally be the lowest cost option but there is a 3 year warranty on all products). They are cheap and will ship anywhere.
For an amp one of their basic own brand swissonic unit would be the cheapest
https://www.thomann.de/gb/swissonic_sa_65.htm it would give you 60W which with 4 speakers per carriage at 2w per speaker would allow for 7 carriages and can run on 24v. They also do a more powerful 120w version that has 12/24v powering ( hhttps://www.thomann.de/gb/swissonic_sa_125.htm )for a few (literally a few!) more quid. (€139/€149)
This would accept any standard microphone with the industry standard XLR connector. My suggestion being the Behringer XMS1800S, metal costruction and super cheap you ger 3 for about £30.
https://www.thomann.de/gb/behringer_xm1800s_set.htm . and for cabling something like this,
https://www.thomann.de/gb/pro_snake_tpm_6.htm , will be fine.
You could, of course, make you own mic panel if required/wanted and there are panel mount microphones and microphone switches available should you want to do that.
As for speakers something like this would be cheap if you don't want it built into the ceiling
https://www.thomann.de/gb/thomann_wsp_135_6.htm , or for in ceiling mounting something like this would be a cheap option
https://www.thomann.de/gb/rcf_pl_60.htm .
For connecting the carriages together you would need 2 wires to go between carriages and as this is 100V you would need connetors that are shielded for touch in both inlet and outlet. A suitable connector would be the weather resistant version of Neutrik's speakON connector with a rain/weather cover, this would allow the female end to be on the carriages and a short male-male cable for the connection between the carriages so it would not matter which way round a carriage is.
This,
https://www.thomann.de/gb/neutrik_nlt_4_fp.htm , for mounting on the carriages and this
https://www.thomann.de/gb/neutrik_nlt_4_fx.htm , for the cables.
The cable that connects this all together needent be anything special (although it might need to be fire rated), just 2 cores that can handle about 2A at 100v, 2 core orange lawn mower cable can usually be used without problems if the cable is ot stressed to much (although check the rules on whats allowed for 100V systems).
The internal speaker to speaker cable can be of the same type as the exterior cable if it makes it cheaper/easier.
Something like this,
https://www.thomann.de/gb/the_sssnake_ssk_215_bk_100m.htm , would be perfect.
If it is decided that a wall mounted volume control would be useful then a variable attenuator such as this,
https://www.thomann.de/gb/apart_e_vol_20.htm , can be used. This model can handle up to 20w.
Lastly a wall mounted speakON connector in each place where the amp may be situated (i.e. in every carriage where the guard may be located) will allow the amp to be moved as necessary and as it will be a parallel 100V system it does not matter where in the consist the amp is located.
Whilst I am not that knowledgeable/experienced with these systems this should give you a starting point. Websites such as soundonsound, and pro sound web should also be able to give you some advice.
And as for cost (excl. battery) for 5 carriages (20 speakers) incl. 1 amplifier, connectors (15 female (2 each on the outside and one in each carriage for connecting the amp) and 9 male (8 for the inter-carriage cables and one for he amp)) and 200m of cable, you would be looking at approx. £910 with wall/ceiling mounted speakers and approx. £890 with in ceiling (recessed) speakers.