Not feasible to have coastal defenses in Davao 1915.Chapter 13: Gunboat diplomacy
Late mid December 1915. There had been a strike that turned into a riot in Davao city, Japanese immigrant workers working in Banana plantations had went into a strike, and upon learning that their demands are not met they went on a protest, which turned violent as looting and destruction of property. The civil guard opened fire and 20 of the strikers are dead. It went out of the news, and Japan responded by sending their battleships to Davao as a show of force as 20 Japanese lay dead after the strike
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Japanese ships on Davao.
In response the Philippines ready their coastal guns and ordered the garrison to ready the artillery and guns in case the Japanese ships might attack or bombard the city. They also sent the navy.
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Filipino artillery regiment near the coast preparing for a possible battle
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Philippine coastal defenses in Davao.
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Philippine Navy ship sent to Davao
It was the Davao Crisis and it lasted a month until the US, and UK intervened, Japan withdrew her forces by January. This event would have a huge impact on the development of the Philippines.
As your POD, population of Davao city was less than 8,000 OTL. 21,000 OTL 1918.
Don't be biased on present day issues . The only places you need to protect back then were Luzon and certain parts of Visayas like Cebu, Panay, Iloilo which got an economy and a lot more people. Even US only bothered protecting Manila from the get go in early 1900s, despite with more cash.
So it is also a matter of cash. Priority will always be where the money is coming from. from your POD that is Manila. Cost of batteries alone is expensive. Cost of batteries would depend on how many inches those guns are. Then you got the concrete, labor cost, maintenance cost.
Focus on development as per your POD. You have too much present OTL foresightHmm how about htat, I placed railways up north for the mines, farms and plantations. and to connect towns, cities. and to connect them to ports for trade
Although your idea is correct, it is also a matter of profitability and capability to pay.
Railroads companies assuming you bid using private companies require profit. If this is totally government funded and government maintained, the government will be bankrupt. The Link I gave you stated 283 miles of railroad total cost was around $12m in OTL 1906. Priority will always be Major cities, Dagupan to Manila, Manila to Legazpi as most the economy is coming from that area initially.
Only until you develop places in time that those areas matter for railroads. Think of it as Return of Investment.
But if you got enough money why not. But that is assuming you have money. Philippines got limited funding, resources upon your POD. max in 1899 would go at $7m range.